There has been no press release yet from Microsoft on this, but the following bullet-points outline improvements for the Xbox 360 HD DVD add-on that are apparently to go in tomorrow according to threads at the AVS Forum by Amir Majidimehr, VP of Microsoft's Consumer Media Technology Group:
  • Fix DD being stuck in "Night Mode" which is essentially Dynamic Range Compression leading to the sound sounding very flat
  • Option to output ALL HD DVD audio as DTS 1.5Mbps (list includes DD+, TrueHD, DTS, and any other audio option offered on HD DVD discs I may have missed)
  • Option to output ALL HD DVD audio as DD 640Kbps, (which it currently does, but will be fixed from the Night Mode bug), list includes DD+, TrueHD, DTS, and any other audio option offered on HD DVD discs that I may have missed.
  • Option to output ALL HD DVD audio as WMAPro 1.5Mbps, (list includes DD+, TrueHD, DTS, and any other audio option offered on HD DVD discs I may have missed) for those receivers that support WMA.
  • Option to enable/disable Night Mode (with the default set to off)
  • Fix a number of disc incompatibilities (including DVD Essentials) as well as a number of Lip Sync issues.
Any forthcoming press release by Microsoft will be posted here when available. Here are a few threads you can track at AVSForum for the latest info: Feedback for May 15th's Xbox 360 HD DVD Audio Update
Xbox 360 HD-DVD add-on update thread