This affects both the radio music and the music that plays in the opening menu. I also have the exact same problem in Fallout 3 when I run it on the same computer(and in fallout 3, as soon as I open the game, you can see my stereo switch between 7.1 audio and stereo right before the opening music starts playing)
I am running an ATI 5450 that is connected via HDMI to my stereo then connected to my Sharp 52" LCD TV. I'm running Windows 7.
I don't have any problems in any other games. I've played Borderlands, Bad Company 2, and Aliens vs Predator with no sound issues.
If I turn off the MPEG-I Stream Splitter codec then the music shuts off in both games and I can play them(and it allows Fallout 3 to stay in 7.1 audio if the music never starts playing). However, both games crash after a couple of minutes of playing them with the sound disabled in this way.
Although both games work fine on my laptop and my other PC, which leads me to believe there is something I can do to fix it on this PC. I just can't figure out what.
