IE9 To Support CANVAS Tag (plus VIDEO, AUDIO, and SVG)
  
    * MP4 H.264 playback support, using hardware or software decoding
    * Support for WebM software is not included in this release

  
    * MP3 and AAC audio support

Somehow, I'm not surprised that Microsoft once again chooses to be different from everybody else. I can encode in Vorbis and Theora/VP8 for Chrome and Firefox, or MP3 and H.264 for IE9, but there's no set of codecs which works on all three major browsers.