Fix MP4 consumer with only audio

pkg/core/codec.go

@@ -52,6 +52,18 @@ func (c *Codec) IsRTP() bool {
 	return c.PayloadType != PayloadTypeRAW
 }
 
+func (c *Codec) IsVideo() bool {
+	return c.Kind() == KindVideo
+}
+
+func (c *Codec) IsAudio() bool {
+	return c.Kind() == KindAudio
+}
+
+func (c *Codec) Kind() string {
+	return GetKind(c.Name)
+}
+
 func (c *Codec) Clone() *Codec {
 	clone := *c
 	return &clone

pkg/mp4/consumer.go

@@ -153,6 +153,10 @@ func (c *Consumer) AddTrack(media *core.Media, _ *core.Codec, track *core.Receiv
 }
 
 func (c *Consumer) WriteTo(wr io.Writer) (int64, error) {
+	if len(c.Senders) == 1 && c.Senders[0].Codec.IsAudio() {
+		c.start = true
+	}
+
 	init, err := c.muxer.GetInit()
 	if err != nil {
 		return 0, err