Other than your sound card being an overkill (not a problem of course, since it's awesome for all types of games. I'm actually envy of you ).
my guess would be drivers issue.
I'd suggest running diablo in compatibility mode for windows xp or windows 98 (if there's an option for win98, can't remember) altough I don't believe that'll work.
It can also be a problem with the game, since you can't expect game programmers to support future hardware, but more likely you will expect
hardware engineers (sound card & graphics card) to use backward support for older technologies.
Since we've went through at least 2 operation systems from diablo 2 (vista, win 7) we can expect some incompatibilities issues
Sorry, I don't think there's anything to do about it