You can turn down electrical toy volume to something more tolerable by adding a resistor to a speaker wire.

Things needed:

Electronic toy to be silenced. The "BEEP BEEP" to be "beep beep".
Tools to open the toy and put it back together. A screwdriver will usually do.
A resistor. More about this later.
Wire cutting and stripping tools. Electronic tools are nice, but a pair of scissors and DIY spirit will do just fine.

The toy, or the electronic device, will have to be battery operated and it must have a speaker. Most electronic toys with non-mechanical sounds are like this.

Example toy here is an R2-D2 alarm clock. By default it makes beeping noises that are a bit too loud. It uses two AA batteries.

SAFETY DISCLAIMER: Warranty will be lost and the toy may break. Tools may hurt you. An opened toy or incorrectly repaired toy may be hazardous. Proceed only if you know what you are doing. Operate only with low voltage toys that use batteries.

Step 1: Dismantle the Toy and Locate the Speaker

Most toys are held together by screws. Loosen the screws and open they toy. Locate the speaker. The speaker is usually round, has a magnet and two wires connected to it. Remember the dismantle process as you will have to put the toy back together later.

In the example case I have dismantled the R2-D2 alarm clock. He didn't seem to mind. I did not disconnect the batteries so the toy is "alive" and can be tested during the procedure. R2-D2's speaker seems to have blue wires. That's nice.

Step 2: Cut a Speaker Wire and Strip the Ends

Pick either of the two speaker wires and cut it. Cut just one wire. Place the cut conveniently so that you have some room to work with the wire ends. Strip the wire ends by removing some of the wire insulation. You should end up with one cut wire with exposed copper threads around the cut ends.

In the example I used side cutters to make the cut. Toy grade wires are usually quite thin, so scissors would have been quite fine as well. After the cut I removed some of the blue insulation by hand. I used my fingernails to break the coating and ripped if off. If you look closely, you can see the rip marks on the insualtion. It's not pretty but it works. Of course you can use fancier tools for this.

As long as the wire ends dot touch each other the toy should be completely silent but otherwise fully operational. I tried it by pressing the toy buttons, looked at blinking lights and listened to the sound of silence.

Step 3: Find a Suitable Resistor

Find a suitable resistor to bring down the volume to appropriate level. Try out different resistance value components. Star with low resistor values first.

A resistor is an electronical component that implements resistance. In this case, it lowers the volume when placed in the middle of the cut wire. Resistors have a resistance value measured in ohms (symbol: Ω). The higher the resistance, the more it turns down the volume here. Resistors can be bought at electrical stores. The pictured set of 400 assorted resistors was US $4.58 on eBay.

In the example case I connected jaw cables to the wire ends and other ends of the cables to a resistor to be tested, starting from low value resistors moving towards higher. The jaws makes this step a bit easier, but you can do without. Simply touch the resistor terminals with the wire ends and test the volume output. I tried 10Ω (no noticeable effect), 22Ω, 47Ω, 100Ω (nice but not quite enough) and finallly 220Ω (pictured) which I found appropriate for this case. This turned the volume down about two thirds.

For something fancier, you may want use an adjustable resistor.

Step 4: Connect the Resistor to the Speaker Wire

Connect the resistor to the previously cut stripped speaker wire. Resistor orientation makes no difference. Solder the joints. Add heatshrink tubing (remember to insert before soldering) or electrical tape (apply after soldering) to insulate the joints.

In the example I was too lazy to take out my soldering tools, so I just wound the wire ends and resistor leads togerher and insulated the joints with electrical tape. For anything more serious I would have used solder and heatshrink tubes. But this is a toy, the joint is well protected inside the casing and it is not exposed to physical stress so tape is just fine. And I got the toy back to working condition much faster. And yes, it's just laziness.

Step 5: Put the Toy Back Together

Put the toy back together and you're done!

R2-D2 turned out just fine, making those beeps as happily as ever - just a bit quieter.

I will attempt to answer any questions you may have.

PS. I have previously tried stuffing toilet paper etc. to speaker area to turn down the volume but that reduces sound quality. Using a resistor is quite more refined and the sound quality seems unaffected.