diff --git a/src/sound/sdl_s.cpp b/src/sound/sdl_s.cpp
--- a/src/sound/sdl_s.cpp
+++ b/src/sound/sdl_s.cpp
@@ -18,8 +18,15 @@
 #include "sdl_s.h"
 #include <SDL.h>
 
+/** Factory for the SDL sound driver. */
 static FSoundDriver_SDL iFSoundDriver_SDL;
 
+/**
+ * Callback that fills the sound buffer.
+ * @param userdata Ignored.
+ * @param stream   The stream to put data into.
+ * @param len      The length of the stream in bytes.
+ */
 static void CDECL fill_sound_buffer(void *userdata, Uint8 *stream, int len)
 {
 	MxMixSamples(stream, len / 4);