--- channels/chan_multicast_rtp.c.328209	2011-09-16 16:54:24.000000000 -0400
+++ channels/chan_multicast_rtp.c	2011-09-16 17:47:24.000000000 -0400
@@ -118,7 +118,15 @@
 	struct ast_sockaddr control_address;
 	struct ast_sockaddr destination_address;
 	struct ast_channel *chan;
-	format_t fmt = ast_best_codec(format);
+	format_t fmt = 0;
+	const char *codec;
+
+	if (requestor && (codec = pbx_builtin_getvar_helper((struct ast_channel *)requestor, "MULTICAST_RTP_CODEC"))) {
+		fmt = ast_getformatbyname(codec);
+		ast_verb(3, "MulticastRTP set codec to %s\n",codec);
+	}
+	if (!fmt)
+		fmt = ast_best_codec(format);
 
 	ast_sockaddr_setnull(&control_address);