feat: add a default audio codec for the HLS video player

This seems to reduce instances of audio garbling when
switching levels during HLS video streaming.

Author: nsprenkle

xmodule/assets/video/public/js/02_html5_hls_video.js

@@ -27,6 +27,12 @@ HLSVideo.Player = (function() {
         // do common initialization independent of player type
         this.init(el, config);
 
+        // set a default audio codec if not provided, this helps reduce issues
+        // switching audio codecs during playback
+        if (!this.config.defaultAudioCodec) {
+            this.config.defaultAudioCodec = "mp4a.40.5";
+        }
+
         _.bindAll(this, 'playVideo', 'pauseVideo', 'onReady');
 
         // If we have only HLS sources and browser doesn't support HLS then show error message.