I think Microsoft should be kicked for not making this clearer doe everyone.
DX10 and DX9.0c (actually DX 9.0L aka DX9.0ex) are completely seperate
installs in Vista. You have both installed on your system. DX10 for the new
games coming and DX 9.0L is for legacy support, which means it's there to
run all the games that still user DX9 (and 8 for that matter). DX10 is a
complete re-write from teh ground up, fixing all the problems DX9.0c
suffered since it was a add-on to an upgrade to an add-on to and upgrade
going back to I believe DX8.
Anyways, you can install all the DX9.0c related stuff you want and not have
any worries about DX10. Probaly the best way I can describe it, is to say
it's like having Half-life 2 installed and wondering if you can install
Half-life 1. Different apps, the OS won't let you overwrite any of the DX10
files.
If go to the command prompt and type DXDIAG you should see that your system
is DX10 like this pic
http://images1.filecloud.com/389200/DX10.jpg
But if you click Display, you see that it refereces DX9Ex as it's DirectDraw
Interface, because that's what Vista uses for things such as Aero. This in
essences proves you're running both DX9L and DX10 at the same time
http://images1.filecloud.com/389199/DX9Ex.jpg
The Audio issue you might be talking about espeically from the Creative side
of the house, is probably more around the Digital Rights Management rules.
Things are "broken" if you will, because DRm requires a secure connection
that won't allow someone to connect up via the SPDIF or whatever to make
copies of Music and to soem degree movies. If oyu do a search for DRM and
Vista there is a fair amount of details (and ticked off people) about it.
The Audio world is moving away from DX and to OpenAL. I don't follow it that
closely, so someone with more experince knowledge would have to answer any
details