Actually you pretty much answered your own question,
My only guess would be the soundcard(when i turn it off i get same fps in Windows 7 compared to XP with sound turned on).
Despite it still being sold according to the Speedlink site, the audio chip on that card (C-Media CMI8378) is close to 10 years old. Found a couple of reviews for cards using it going back to 2002-2003. Speedlink may even have a compatible Windows 7 driver
, but inside the zip file they have is also the drivers
for XP and Windows 98 (with the Windows 98 driver dated from 2004).
That you say the game performs as expected when you disable the sound says to me that it is the sound card and its drivers that are the problem, not your video card or a combination of your CPU and motherboard.
A couple of things to try,
1. What are your in game audio settings? Have a look here, TweakGuides.com - Unreal Tournament 2004 Tweak Guide
that card of yours would fall the second or third option, and definitely not the fourth.
2. Go here, Downloads
and download and install the oalinst. That will install the OpenAL files, which with your card will make use of the software renderer portion and not the hardware version. Then try the game with the "System Driver" option checked and unchecked to see if it makes any difference.