I am afraid this will be quite hard to solve... since we don't have the system to check.
Let's make a little experiment... start regedit, go to
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunServices
select it and export the selected branch (for backup purposes). Now, select the value avast! on the right side and delete it.
Then, do the same for the key Run (instead of RunServices) and its value ashMaiSv (if present).
This should disable avast! completely. So, after you restart your computer, avast! will not be loaded.
Now, you can check if your speaker system is working properly. If yes, go to the avast! installation folder and run ashServ.exe. This will start avast!. Now, the speaker system should stop working... is it correct?
To put avast! registry entries to the original state, you can simply import the two files you have exported at the start.
The second thing you can try it renaming the file aswmon.vxd in the avast! installation folder to some other name (instead of removing, you can also move it anywhere else). This will completely disable the Standard Shield provider (that is the one most likely to cause the conflict - if there really is any conflict). After reboot, Standard Shield will not be loaded. Does your speaker system work in this case?