Well, I use a SoundBlaster Live! MP3+ card myself and before that I used a Yamaha OPL-3Sax which I still own, just in case I decide to come up with another computer.
I feel like onboard sound is really ok when you're aiming just at listening to a movie or a song or something and you don't really care for the real quality of it.
I like sound quality both in game and music, and I don't have many complaints regarding IL-2's sound quality. As it turns out, this thread just reminded me to install the sound card onto the new computer
By the way, I've been away from the IL-2 (or general gaming) community for a while, and I feel kind of shocked when I read news about sound hacking/cheating and stuff. Could anyone bring me up on that via PM? I feel it's something of a no-no commenting about it in the open.