For me, ALSA creates a common ground for such layer to arise, so I
just suggest to wait for ALSA to mature and more apps to support it.
And, in fact, I don't know about you, but all I do is:
* Install my Debian image
* Create a profile for me
* Install UT2004
* And play with sound and graphics in all glory
Sound chip is discovered with small app discover, similar to Kundzu,
without bothering me (all I can discuss about is ALSA default mixer
settings, but it can be saved in some kind of default for common user
- if you wish). Graphics is detected too - it still needs lot of work
but it's getting here.
So it works, without any kind of problem.
As for multiple sources of sound played in once - I don't know, I
don't like a Windows solution for this, but some kind of such level
mixing would be nice - but it could be even better than other OSes,
for example, when I watch a movie, all 'dingles and dangles' (new
email, friend bothering you trough ICQ with important message,
download completed) could be very silent, somewhere in the background.
It could be done trough D-BUS, I guess - as I know that there was
ideas of while watching something full screen, trough D-BUS get other
apps like GAIM know that I'm busy now watching movie :)