You didn't mention which skin you're using, one of the "default" ones or one of Craftec's great contributions.
If you're using one of Craftec's skins, then (and I could be mistaken, feel free to correct me, Craftec or whoever) if you disable avast sounds then you get the "custom" sounds built into the skin. In one of the default skins, disabling sounds simply turns them off, as expected.