I could be wrong, but ASFIK, two devices using the same IRQ in Windows XP is not a problem in most cases, but I think it depends on the devices. It was only a major problem in previous versions of Windows.
You're right, it would seem a bit strange that the freeze only happens in Multiplayer, but I had onboard sound and it caused major stutters in Half-Life 2, Day of Defeat: Source, etc. I got an Audigy 4 and the stutter disappeared.