Even when the server "is" slow and there is a lot of text lag, my onboard sound still works, although it can slow up the lag even further if used a lot, but doesn't disappear or cause a CTD, so its probably due to a combination of things.
It's more than likely relatd to individual setups, where the sound card interupt is sharing interupts with some other major hardware system such as the video or network card. Sound hardware can share PCI Steering without problems, but its recommended to get the sound interupt otherwise separate from any other major hardware, else this problem seems to be very common in some games. (To an extent it happened in the old Air Warrior game also, those having major vox problems usually had sound sharing an interupt with a network card or with video whether AGP or PCI)
Depending on the bios fitted, one can either assign a specific interupt to sound (ignoring PCI steering which shouldn't be a problem), or if there is not that facility in the bios, to swap PCI internal cards fitted around into different slots by trial and error, updating the ACPI system each time to re-assign interupts in the Windows OS on reboot. XP seems to be particularly bad as quite often sharing the sound with other hardware, without the choice of the user.
Some have oversome this in XP by disabling one or both of the comm ports in bios if it not in use (say they have a cable card for example), so that another interupt/s becomes free for the available pool of interupt resources.
Just some thoughts. My sound interupt "is" separate from video, network card, etc and is only sharing with the PCI steering host.