Overtone provides two simple functions to allow you to record audio: recording-start and recording-stop. These are used as follows:

(recording-start "~/Desktop/foo.wav")
;; make some noise. i.e.
(demo (pan2 (sin-osc)))
;; stop recording
(recording-stop)

You'll now see a file at ~/Desktop/foo.wav which will contain the audio you generated.

Saving buffers

If you have a buffer containing audio you wish to save to disk, you can do this with the buffer-save function:

(buffer-save b "~/Desktop/bong.wav") 

Recording with external audio in

(definst auidio [] (sound-in [0 1]))