DirectX isn't fully supported - at least, not by Microsoft - on any platform other than Windows, AFAIK. Certainly not on Linux. Winex might be able to handle it, but that's about it.
Anyway-
1) There'd be more Win32 only code. This would needlessly clutter it up, add more possiblity for OS-specific bugs, etc.
2) It's probably going to be slower because DirectShow would have to be initialized. That might let us do a little onboard hardware acceleration with sound, but not much.
3) The only person who has experience with DirectShow is RT
4) OGG code, in some form or another, has been in since 3.6.5. It's been declared fixed 2 or 3 times, at least, but has only started working for me (reliably, anyway) with the last fixes. Not one campaign, AFAIK, has attempted to make use of it besides TBP.
5) OGG is simply, IMHO, the best format out there. It's open source, there's no licensing or patent issues to worry about, it has better compression quality than MP3 at equivelant bitrates, and it's fully, natively, supported on Linux, Mac, and PC. Adding a DirectShow decoder would just end up causing more Windows-only campaigns, where sound effects and music would only work under windows. Or, people would have to go to the hassle of getting DirectShow up and working on whatever platform they use.
6) With OGG, it's drop in and go. No hassle or fuss with codecs. The only thing you have to worry about is a CTD because of some kind of bug.
