media_stream: set time base for audio on update

Time base for audio should always be the inverse of the sampling rate.

Change-Id: I335a4bab1c29b5f0411dcb38a3bab379b48fa433

src/media/media_stream.h

@@ -126,6 +126,7 @@ struct MediaStream {
         } else {
             sampleRate = f->sample_rate;
             nbChannels = f->channels;
+            timeBase = rational<int>(1, f->sample_rate);
         }
     }
 };