PCM. HD-DVD's low transfer rates can't handle it. Actually, I think there's a mistake in there, because a "mandatory" format is one which, if included, eliminates the need for another : and I believe that either DD+ or dts-HD fits that description, if only because [as might be] it is the base-profile levels, DD AC-3 and dts Zeta, which have been selected as mandatory.
Chris dac is correct...mandatory means that if it's there, you don't need anything else (ie, a DD+ track on an HD DVD title would be good enough, no additional audio track needed).
What is truely optional is whether these new codecs are fully supported to be handled/playbed back in the playback hardware.
HD DVD does have a better deal here IMO, in that all players are required to play DD+ and DTHD signals (and I think DTS HD) whereas BD players are *NOT* required to and only have to provide support for LPCM and legacy DD/DTS codecs to meet spec.
Now...before we all cry the blues...let's realize that just because something isn't officially "required" doesn't mean that it won't become standard on players in a few short generation cycles.
Is DTS support, component video, or progressive-scan "required" on a DVD player? Nope. Yet almost all players...even el-cheapo players...give you DTS and component video...and many affordable players do progressive-scan.
So the real question is after 2-3 product turns...what are HD DVD and BD players doing *then*? If by then you've got full-audio support on BD players then we're fine. If Dolby TrueHD, DD+ and DTS-HD support are only showing up on a few choice high-end Blu-ray players, time to take the manufacturers to task...
BTW, HD DVD "required" support only means the player has to produce sound...not necessarly preserve full fidelity. As an audiophile, listening to 2-channels of a 5.1 DTHD mix or a lossy 1.5 mbps DTS transcoding of a high-bit-rate DD+ soundtrack isn't exactly going to satisfy either.