Bike trailer. Not all trailers may be suitable, check the next step.
1/2" Plywood (2' x 3' piece or so)
2 x 12V, around 15Ah sealed-lead-acid (SLA) batteries (see below)
24V battery charger (see below)
24V amplified speakers (see below)
3.5mm audio cables and Y cable
Small stereo mixer (see below)
mp3 player holder
lots of 1/4" bolts, spacers and washers
15' of 16 gauge wire
electrical crimp connectors (home improvement and auto parts stores have these)
10A circuit breaker
see next step.
I'm using 2 x 12V, 18Ah capacity SLA rechargeable batteries (also known as "sealed lead acid" or "gel cell"). Together this is about 1/2 the weight of a car battery and it powers my setup for 8 hours at full volume. Reducing the sound volume just a little it will last much longer. You can use bigger or smaller batteries depending on how long you need to run for. SLA rechargeable batteries are very inexpensive and come in lots of sizes. You can get new ones like mine for $35 each - try batterymart.com or batterywholesale.com or amazon.com. You're looking for "12V SLA battery" or "Sealed Lead Acid battery". This type of battery is often found at recycle centers in good condition, so you can get them for free that way - look for discarded computer back-up power systems. You can use a lithium battery for even less weight, but it will cost around $400 for the same capacity.
These are mass produced for 24V scooters so they are only about $15. Amazon.com has lots of them like this one
, just search for "24v scooter charger". A 2A charger is perfect here.
24V AMPLIFIED SPEAKERS
Using amplifiers that run directly from your battery (without an inverter) makes your setup a lot more efficient. I have instructions on how to build battery-powered amplified speakers here
. We sell those same speakers fully assembled at MonkeyLectric
parts-express.com and amazon.com have all the ones you need.
POWERED STEREO MIXER
To get full volume from the setup you need a small powered stereo mixer. The mixer will also let you plug in a microphone and additional sound sources so its convenient. I used a Rolls MX51S, you can get this from parts-express.com or amazon.com, it does the job for about $55. There are some other brands with similar models.