I've been living with this problem since I first puchased Dungeon Siege. I just figured out what's causing it now I want to see about fixing it. The issue is that EAX starts to lag and do all kinds of strange delayed and phantom sound effects during gameplay. It takes awhile for this to occur, 10~15 minutes or so and there it is. I finally decided to turn off EAX in the game options and it worked properly. Of course I had to also turn the speakers back to 5.1 again for some reason in the Speaker Control Panel Up[censored] all the device drivers has not resolved this now has a clean install of XP Pro.
 
Here's my system specs for my gaming rig:
 
Aopen AX4C Max II motherboard with bios v1.12b
P4 3.0c with a Zalman Copper Bloom
Apacer DDR400/PC3200 CL3 (2 x 512MB)
NuTech 8x DVD-RW
TDK Velo CD-RW
Segate 80GB SATA (2 drives in a RAID 0 Array on the ICH5R)
Celestica Radeon X800 Pro
Aopen H600 Chassis with extra case cooling fans
Channel Well Tech ISO-500D Power Supply
 
Any tips and suggestions would be appreciated thanks