fix audio

av/av.go

@@ -238,6 +238,7 @@ type Packet struct {
 	Idx             int8          // stream index in container format
 	CompositionTime time.Duration // packet presentation time minus decode time for H264 B-Frame
 	Time            time.Duration // packet decode time
+	Duration        time.Duration //packet duration
 	Data            []byte        // packet data
 }