The possibilities are, depending on platform and compilation options:
.RS 8
.TP
+.B pulseaudio
+PulseAudio.
+.TP
.B alsa
ALSA.
Linux only.
.TP
.B query
Causes the string "running" to be sent back.
+.TP
+.B getvol
+Print the left and right volume levels,
+as two decimal integers between 0 and 100,
+separated by a space.
+.TP
+.BI "setvol " left " " right
+Set the left and right volume levels to the given decimal values,
+which should be between 0 and 100;
+echo back the new values as for
+.B getvol
+above.
.PP
Other commands are ignored.
After the first command the connection is closed.