I have recently read (and posted) some other topic about sounds and Avast not working as it is supposed to work, but no actual resolution was found.
[
OT] and JFYI, installing SP3 "
shouldn't" request more resources than SP2, and there are reported cases here in the forum where Avast should be removed (using the Clean Uninstall utility from Avast) before updating to SP3 (related to changes in Windows Validation/Registration after the SP3 update). In any case, I have no idea if updating to SP3 is going to resolve your problem.
[/
OT]
Being at least the second case, I think that someone from Avast Team should come to this topic to provide some specific technical guidance (so to solve the problem for "all" users/cases).
EDIT_UPDATE: I think the other topic was:
http://forum.avast.com/index.php?topic=81521.0