Hey, I used to have a Diamond PCI card somewhere.
I would say that all bets are off with the drivers, but there is a strong possibility that you could be hearing the problems with the sample rate converter in Windows 7 (and I think in Vista). This affects recording and playback, and the first line of defence is to make sure that the source audio's sample rate is the same as the "default sample rate" that is set in every area in the Windows Control panel audio area.
The sample rate converter built into XP has a range of settings that are usually set to "high quality", and sample rate conversion is rare anyway with that OS. With Windows 7, the concept of the newer Windows mixer irequires that the sample rate converter changes all the audio on the fly to the default ample rate, but the converter is not very good. This doesn't always affect the audio - anything using DirectX seems OK, but anything using "WaveOut" audio (eg YouTube playback) is affected.
Microsoft acknowledges the problem and have issued a patch at http://support.microsoft.com/kb/2653312
Thanks for the KB link. I'll take note of it, although prior to my seeing that message, I found there is a newer mobo chipset driver available. I downloaded that and installed. Initial tests seem to show the problem is cured. YouTube streaming, or just audio played through Media Player (or my other system playback programs) are recording onto Audition cleanly with my first tests.