FS-7083 #resolve #comment this should do it. The problem is linked to side-effects from the read thread being delayed by writing to the file handle. It was so much worse on mp3 because the shout encoder blocks while its churning the data and delays it more. This patch adds a dedicated thread for writing to the file and the channel_variable RECORD_USE_THREAD=false will disable it and sync may still be maintained at the cost of dropping more data from the audio signal.

src/switch_time.c

@@ -1046,6 +1046,8 @@ SWITCH_MODULE_RUNTIME_FUNCTION(softtimer_runtime)
 				tfd = -1;
 			}
 		}
+
+		if (tfd > -1) MATRIX = 0;
 	}
 #else
 	tfd = -1;