Menus/Sound Options: unmute if slider is adjusted

- Master volume: unmutes both SFX and music (if both were
  muted)
- SFX, music: unmutes the individual channels

src/menus/options-sound.cpp

@@ -97,19 +97,11 @@ struct Slider
 
 	void input(INT32 c)
 	{
-		if (c == -1 && &volume_ == &cv_mastervolume)
+		M_ChangeCvarDirect(c, &volume_);
+
+		if (!toggle_(false))
 		{
-			// Master volume does not necessarily change when
+			toggle_(true);
-			// music or sound volumes change separately. So
-			// cv_mastervolume could still have its default
-			// value, and M_ChangeCvarDirect would do
-			// nothing.
-			CV_Set(&cv_digmusicvolume, cv_mastervolume.defaultvalue);
-			CV_Set(&cv_soundvolume, cv_mastervolume.defaultvalue);
-		}
-		else
-		{
-			M_ChangeCvarDirect(c, &volume_);
 		}
 
 		shake_ = !shake_;