FIX(client): Prevent local muted users from triggering attenuation

When 'while others users talk' attenuation is enabled, unmuted users
still trigger attenuation even if they are locally muted.

The fix is don't add local muted users' audio source to the mixing
for the audio output

Fixes mumble-voip#6247

src/mumble/AudioOutput.cpp

@@ -453,11 +453,13 @@ bool AudioOutput::mix(void *outbuff, unsigned int frameCount) {
 		if (!buffer->prepareSampleBuffer(frameCount)) {
 			qlDel.append(buffer);
 		} else {
-			qlMix.append(buffer);
-
 			const ClientUser *user = it.key();
-			if (user && user->bPrioritySpeaker) {
-				prioritySpeakerActive = true;
+
+			if (user && !user->bLocalMute) {
+				qlMix.append(buffer);
+				if (user->bPrioritySpeaker) {
+					prioritySpeakerActive = true;
+				}
 			}
 		}
 		++it;