This just started today. When the virus database receives an update, the voice and ding happen with no popup. Then a few seconds later, the same voice and ding but this time with the popup. This is on Win 7 64 bit.
This has also been happening to me with Windows 7 32 bit.
Many such topics already on this issue, this is just one of them http://forum.avast.com/index.php?topic=101278.0.