If one wants to take an even CHEAPER, simpler route and doesn't mind damaging their speaker cabinets, you can simply buy a set of pannier rack hooks for just a few dollars instead of buying $60+ worth of saddle bags you might not have, and screw them near the top inner edges of the cabinets, but make sure that you also build a strap to pull the speakers down taut because even with the bottom straps on saddle bags, the speakers will want to jump off your rack if you hit a hard bump. Forget about even trying to jump off a curb.
I already had the basket and bungee cords on the bike for carrying items along with the Mission M71 speakers collecting dust and the Coby CD player too when I decided to build the system and only needed the saddle bags and Sonic Impact amplifier. it only cost me $30 for the generation 1 amplifier. The newer one's look better and have really cool looking blue illumination rings around the volume control, but cost $50+.
To make sure the system was doable, I had to first measure my speakers to see if i could find a saddle bag they'd fit in. I hadn't seen $4 rack hooks yet at that point. I brought the dimensions into a local bike shop along with a ruler to check dimensions and the "Banjo" bags you see were adequate for the job.
You could also build your own wooden speaker holders too if you wanted to go with larger speakers etc. If you don't have speakers yet, and don't mind fabricating custom mounts, I'd highly recommend buying a pair of Pyle Pro PDWR63 outdoor speakers. They're lighter than wood cabinet speakers, weather rated, have 6 1/2" woofers, get excellent owner reviews (except for bass ouput), are available for under $115 a pair, and most importantly, are rated at a NICE 94dB efficiency! That's like getting an amplifier that's twice the power with most 90dB or less speakers for free. Every time you add 3 dB to a speaker's efficiency, you get the equivalent of doubling amplifier power. That makes a big difference too when your amplifier only puts out 10-15 real world watts like the Sonic Impacts do.
Once I knew everything would work, I bought the bags and ordered the amplifier. While waiting for the amp to arrive, I test fit the speakers and didn't like the way they sat low in the bags and would rattle around so I went to a local department store and bought an $8 foam rubber camping mat and cut it apart to make a liner for the speakers to securely position them. I needed the box cutter, a pen for marking, duct tape to connect the pieces, and a yardstick for measuring, marking and cutting straight lines.
I had to use the foam plugs that came with the speakers because their ports are in the back (bottom) of the speakers. I also had to cut an opening of the foam lining at the bottom of the bag to allow the speaker wires clearance. I used two layers of foam on one side as three total were needed. Looking down on the liner, it's shaped like a "U" with the open end facing the outside of the bike with another piece filling the inside of the "U" on the bottom. The outside piece of foam isn't attached as it was too hard trying to get a one piece liner to fit. It's important if you build a system like this that you put the separate piece on the outside because the speakers like to push it up and out of the bag on the inside.
Once I got the proper fit for everything, I connected the speaker wires and ran them up the insides of the bag. I soldered both ends of each cable to prevent fraying, particularly the ends going into the amplifier's spring terminals. BTW, the original model's spring terminals were on the flimsy side and have since broken. I haven't seen the back of the newer model yet. I'd highly suggest that if you have a unit with flimsy terminals that you build an external speaker connector that stays permanently attached to the amplifier. Then you could use banana plugs, spade lugs or whatever method you prefer to connect your speakers. Banana plugs would make mounting and removing the system every time you use it go quicker though.
I made each speaker cable a few feet longer than they would need to be in case I wanted to take they system off the bike and separate the speakers on the ground. I just tuck the extra length inside the bags until the speaker wires go straight to the amp. Putting speakers on the ground (in a corner is even better) couples them to the surface(s) and gives you a little more bass along with better stereo separation.
Once the speakers were assembled and ready to go, it was just a matter of plugging everything in more or less once the amp arrived. Not knowing that it came with it's own 1 foot long 1/8" male to male coiled cable (PERFECT for this setup), I bought another longer one cheap at a PC store. I wanted to be ready to roll the day the amp arrived. I'd already bought 2 packs of rechargable NiMh batteries and had those ready to go.
There's a little bit of a squeeze with the basket between the speakers and it has to be removed so that the speakers can be mounted which is a little bit of a pain. The same bungee cords that hold the basket in place serve the second purpose as a shock mount for the CD player which works nicely. I used velcro tape wrapped around the bungee cords and on the back of the CD player which works fine. It holds the player securely while allowing for quick set up and removal. I used the "furry" tape on the player so as not to affect it's use off the bike. The plastic teeth would collect all sorts of lints etc. if I tried going anywhere with the player. Combined with the player's anti-skip memory, the system only skips when the batteries are running low or I'm riding on a really bumpy pothole ridden road where the player doesn't have time to recover from multiple individual skips.
Obviously, an mp3 player won't skip at all, and if you have certain models of iPods, you can even get UF (radio) remotes you can attach to your handle bars with a little work and there used to be models I've seen made specifically for bar mounting. I have to reach back to manually change tracks or volume with this setup, but it's down and dirty, cheap and works.
Even better than iPods though, I'd suggest getting Creative Labs players as they're famous for having superior fidelity and for best sound, don't use mp3s at all, use uncompressed WAVs taken directly from CDs with freeware like Audiograbber. With some players having up to 32gb now as well as the ability to swap flash cards, you can fit close to 20 X 80 minute CDs worth of music in a player with unlimited numbers of additional tracks on cheap flash cards. If I had Windows XP, I'd get a Creative Labs player myself. Going the CD route was a necessity for me.
So how does it all sound? Pretty darn good! Much better than a boom box! It sounded even better when my Sony CD player was still working until it died right after my warranty expired! Of course it can't compete with a 50wpc JVC Kaboom Box with it's two 6 1/2" woofers and foot long ports in the bass and volume departments, but it has better clarity and even at full volume will play twice as long as the 4 hour battery eating JVC. Don't even think about trying to cruise by a nightclub with a system like this because you'll get drowned out big time to the point where you can't even hear your own music, but in a quieter area, you can easily be heard from up to 100 feet away and partiers go nuts when they see you riding by playing good music that doesn't sound distorted.
There you have it. Here's a super easy to build system ANYONE can put on their bike with the absolute most basic tools and get "high end" sound. Despite having just 10wpc and 89dB efficient speakers with puny 5 1/4" woofers, I've felt the system making my bike vibrate when I stop and get off. It's only in noisy areas that it doesn't seem so loud anymore. How much bass boost you use will also affect how loud you play too. This CD player has no bass boost so it will play louder than I used to with the Sony, but at close range, it sounded sweet with more bass, particularly because the bass was punchy and undistorted with the ports plugged. I'm not a fan of boomy, resonant ported bass. I'd rather it be tight and clean even if it's not as loud as in acoustic suspension (sealed box) speakers.
Like I said, if you can do some woodworking, those Pyle outdoor speakers would be lighter and play louder and if you can handle a bit more of wood working and some basic electronics assembly, 41hz makes some excellent sounding 18 volt 50wpc class D amp kits. With the Pyle speakers and more than twice the power, you could get some serious SPL out of a bike system. I'd use larger D cells on a system like that for longer run times. I'm currently working on a 500 watt trailer system that will have lighting effects for next spring, but after I do that, might just DIY a Creative Labs/41hz/Pyle system for cruising. Class D amplifiers, by the way, are almost twice as efficient as "old school" class A/B amplifiers which is especially important for battery powered systems.
If this is all interesting to you, stop by my street party bike website where I'll be posting pics of the trailer when it's done. I'll also be posting a DETAILED step by step DIY for that here as that involves a lot of fabrication. Along with my two bike systems there, I have a links page with a bunch of different systems built by others including Instructables' own wild Choprical Fish for inspiration or just plain oohing and ahhhing.
I haven't updated the site in a while as I'm not really doing anything with street partying at the moment, but WILL do some updating by spring time for sure. I also want to add a section including gear that's good for bike stereos like the stuff mentioned here and more along with another section of theoretical systems I've sketched including a trike system and a Sun EZ recumbent system. Stop by and take a peek using the orange link below if you want some more ideas. You can also take a look at the PARTS at least for the trailer system there too until I update it with the full build and final product.
Street Party Bikes