diff --git a/src/s_sound.cpp b/src/s_sound.cpp
index 2626c2b82..d1c313967 100644
--- a/src/s_sound.cpp
+++ b/src/s_sound.cpp
@@ -1683,7 +1683,7 @@ bool S_GetSoundPlayingInfo (const FPolyObj *poly, int sound_id)
 //
 //==========================================================================
 
-bool S_IsChannelUsed(AActor *actor, int channel, int *seen)
+static bool S_IsChannelUsed(AActor *actor, int channel, int *seen)
 {
 	if (*seen & (1 << channel))
 	{