--- main/format.c	(Asterisk 13.4)
+++ main/format.c	(working copy)
@@ -320,9 +320,19 @@
 void ast_format_generate_sdp_fmtp(const struct ast_format *format, unsigned int payload, struct ast_str **str)
 {
-	if (!format->interface || !format->interface->format_generate_sdp_fmtp) {
+	const struct ast_format_interface *interface = format->interface;
+	
+	if (!interface) {
+		struct format_interface *format_interface = ao2_find(interfaces, format->codec->name, OBJ_SEARCH_KEY);
+		if (format_interface) {
+			interface = format_interface->interface;
+			ao2_ref(format_interface, -1);
+		}
+	}
+	
+	if (!interface || !interface->format_generate_sdp_fmtp) {
 		return;
 	}
 
-	format->interface->format_generate_sdp_fmtp(format, payload, str);
+	interface->format_generate_sdp_fmtp(format, payload, str);
 }
 
--- res/res_format_attr_opus.c	(Asterisk 13.4)
+++ res/res_format_attr_opus.c	(working copy)
@@ -135,5 +135,5 @@
 
 	if (!attr) {
-		return;
+		attr = &default_opus_attr;
 	}
 
@@ -181,10 +181,18 @@
 	if (!jointformat) {
 		return NULL;
 	}
 	attr_res = ast_format_get_attribute_data(jointformat);
 
+	if (0 == attr1->maxplayrate) {
+		attr_res->maxplayrate = attr2->maxplayrate;
+	} else if (0 == attr2->maxplayrate) {
+		attr_res->maxplayrate = attr1->maxplayrate;
+	} else {
+		attr_res->maxplayrate = MIN(attr1->maxplayrate, attr2->maxplayrate);
+	}
+
 	/* Only do dtx if both sides want it. DTX is a trade off between
 	 * computational complexity and bandwidth. */
 	attr_res->dtx = attr1->dtx && attr2->dtx ? 1 : 0;
 
 	/* Only do FEC if both sides want it.  If a peer specifically requests not