Hi,
I’ve had this happen yesterday. (28th June 2014) I shut down the laptop and while it was on the “shutting down” screen a voice message said “Welcome To Avast” I had not installed a new version, but when I turned the computer back on, after scanning with MalwareBytes, and Windows Defender (the Windows 7 version not the full anti-virus version), I opened Avast and noticed there was a new version of Avast available.
I can’t be sure the update notice wasn’t there already (I hadn’t looked) but I’d not seen any Avast! pop-ups saying a new version is available.

Could the “Welcome To Avast” voice message be related to the new version available notice in the update section of Avast! UI?

Is there a possiblity this voice message on shutdown is caused by malware pretending to be Avast! or a similar type issue?

Windows 7. 64 bit
Avast Free
Avast Version was 2014.9.0.2018 is now 2014.9.0.2021

Thanks