sb-simple-audio is a library that provides a simple interface to audio I/O. It is limited to SBCL and Linux (using OSS).

An example:

(defparameter *volume* 0.05)

(defun single-tone (frequency)
  "Sounds a tone of the specified frequency, indefinitely."
  (with-open-stream (stream <B>(sb-simple-audio:open-audio :sample-rate 44100 :direction :output)</B>)
    (let ((phase 0.0d0)
          (phase-increment (* 2 pi (/ frequency (sb-simple-audio:sample-rate stream))))
          (amplitude (coerce (* *volume* <B>(sb-simple-audio:max-amplitude stream)</B>) 'double-float))
          (offset <B>(sb-simple-audio:offset stream)</B>))
         (setf phase (mod (+ phase phase-increment)
                          (* 2 pi)))
         (write-byte (+ offset (round (* amplitude (sin phase)))) stream)))))
(single-tone 600)

It could for instance be used for playing and recording music or in any other kind of audio signal processing.