I recommend getting the Release Candidate and get rid of the Beta version. There are a LOT of bug fixes.
I agree that it's likely a driver issue. Installing Win 7 RC may even solve the issue.
Also, Operating Systems tend to install basic drivers that may not have full functionality.
You need to find out the EXACT sound card/chip that you have and install the drivers for it. If you have onboard sound I'd expect that Windows 7 RC will likely install up to date drivers.
You might wish to consider a dedicated audio card. I'm using the Auzentech Forte (PCIe x1) and I absolutely love it. I also have good M-Audio AV40 speakers. Good speakers are really worth it as they should last dedades if not my entire life.
You can use Vista drivers sometimes for Windows 7 but I imagine you will need 64-bit drivers. Check your motherboard support/download site first.