If it's audio you're after rather than data, then you don't actually need the Arduino's processing ability, as you're just using it for encoding/decoding at either end.
In that case I would just hack a "family communications" radio, or build a few FM radio transmitters...
For a couple of channels, straight radio will be much simpler, more reliable, and have potenitally much better sound quality as you won't need to A/D and D/A convert the audio.
You could try hacking one of those "family radio" communications units, although the audio will be of limited range (about 100hz-3Khz, like a telephone). Do a google search for "family radio service".
You can also build small, stereo FM low power transmitters that have full "deviation" (the modulaiton of the radio signal in FM) and give you something like 100hz-15Khz audio. Then, on the inside end you can digitize and MAX/MSP/PD the audio to your heart's content. the only potential downside to a transmitter kit is that you need to follow good RF design practices: shielding, grounds etc, especially if you are sending three or four channels of audio. Four independent channels (two stereo transmitters) can be done for less than $100.
Check out these FM transmitters: http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=FM10Chttp://www.hobbytron.com/MiniFMTransmitter.html