For the amplifier side, you might consider a few (3 stereo amps for 6 separate channels) chip based t-amp like the dayton dta-2. They're nice and small, DC powered, and at least in the case of the dta-2, the volume and power switch are on a separate daughterboard, which would make interfacing with the Arduino more convenient. As far as the audio source goes, I'm sure it wouldn't be difficult to find some tiny flash based mp3 players for under $10 each.