Fix NOMIXER compiling

src/sdl/sdl_sound.c

@@ -1434,6 +1434,8 @@ static void I_ResumeGME(void)
 
 boolean I_LoadSong(char *data, size_t len)
 {
+	(void)data;
+	(void)len;
 	return false;
 }
 
@@ -1495,6 +1497,7 @@ boolean I_FadeSongFromVolume(UINT8 target_volume, UINT8 source_volume, UINT32 ms
 	(void)target_volume;
 	(void)source_volume;
 	(void)ms;
+	(void)callback;
 	return false;
 }
 
@@ -1502,6 +1505,7 @@ boolean I_FadeSong(UINT8 target_volume, UINT32 ms, void (*callback)(void))
 {
 	(void)target_volume;
 	(void)ms;
+	(void)callback;
 	return false;
 }