Make audio bus channels' peak volume consistent

Channels that are inactive -or when playback has not started yet- will report -200 dB as their peak value (which is also the lowest value possible during playback).

servers/audio_server.h

@@ -199,7 +199,7 @@ private:
 				last_mix_with_audio = 0;
 				used = false;
 				active = false;
-				peak_volume = AudioFrame(0, 0);
+				peak_volume = AudioFrame(AUDIO_MIN_PEAK_DB, AUDIO_MIN_PEAK_DB);
 			}
 		};