See these threads, maybe you’ll find the solution:

http://forum.avast.com/index.php?board=2;action=display;threadid=5110
http://forum.avast.com/index.php?board=2;action=display;threadid=4464
http://forum.avast.com/index.php?board=2;action=display;threadid=2120

Can you disable avast! sounds in the program settings and see if it helps?