If there aren't any *.wav files into your avast folder installation (generally, <avast>\English\*.wav), you need to repair your avast installation.
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair). You must be connected to the internet while repairing.
If this does not help, can you uninstall / boot / install / boot again?