I have sshd and rsync on my z now, so I can easily rsync files to a 256MB CF card. The first mp3 files I sent wouldn't play with the standard player program, so I tried making constant bit rate files with "lame -b 80 infile.mp3 outfile.mp3". These files are smaller due to the low bitrate, and they play fine.
In the San Francisco bay area, the radio spectrum is very busy, so it's hard to find a quiet spot to broadcast from the little transmitter. I'm on 87.7FM for now, but on the high bridges (where I presume the real 87.7 signal is least obstructed), I can barely hear my own signal. Everywhere else, there's a little white noise but it's quite tolerable. I am considering compressing my audio waveform as I down-convert it so I can push through the noise just a little bit more. On the freeway, though, I rarely notice the noise.