How to get decent-quality sound recordings into Asterisk from a Mac without a ton of work:

1. Record with GarageBand. Use a real microphone, not the one built into the Mac. If your Mac doesn’t have a microphone jack, consider buying a Griffin iMic.
2. Export to iTunes. With GarageBand 1.0, this seems to be the only export option available.
3. Find the track in iTunes and convert it to an MP3. This shouldn’t be necessary (or really even a good idea), but my copy of sox (below) couldn’t handle the AIFF file that GarageBand produced.
4. Run sox with these options: sox recording.mp3 -r 8000 -w -s -c 1 recording.wav resample -ql
5. Verify that the WAV file sounds okay.
6. Copy the WAV file into /var/lib/asterisk/sounds. You can now use it with Asterisk’s Playback application.

The WAV file produced is sampled at 8 kHz, with 1 channel of 16-bit signed linear audio. This seems to be the best format for Asterisk, assuming that you don’t mind using around 16 KB/sec for audio files.