Since then I have done a couple of upgrades. Bought a small MP3 player mainly to cut down on the bulk and now have rigged to be "cordless".
First thing is to have a set of FRS radios that you can program to take the confirmation beep off - most of the newer ones have that. In the speaker box, use a 1/8" plug double ended cable (6 foot length Radio Shack stock#42-2387A) from the mini amp to the headphone speaker jack on the radio. This is now your receiver.
Attach your CD player / MP3 player with another 1/8" double plug cable to the microphone jack on the second FRS radio. You will likely find that it is a samller diamater jack on the radio and you will need to use a reducing plug from 1/8" to 3/32" plug - I bought mine at the "source" - part # 2740381 - have also seen the same reducing plug at Future shop - usually will find them in the cell phone accessories. Anyways, this unit is now your transmitter.
So, turn on the music source, press transmit on the FRS and it will be received and transmitted from the caller unit.