Update. I just did a check on the audio formats being used and I can confirm that for PAL releases all Blu-Ray discs I just looked at are using the old 5.1 Dolby Digital audio standard and HD-DVD are using the new Dolby Digital Plus or they do not have the standard listed.

So for audio it does look like HD-DVD is currently the better option. Another reason to wait longer to find which of the two formats will win the battle and if Blu-Ray releases will eventually start to support the better Dolby Digital Plus format.