Hello,
The Avast audio was too loud for me, and found it can be adjusted in the standard Windows Volume Mixer.
After an Avast audio is played it shows up in the built-in Windows Volume Mixer for a while.
You probably need to have "All devices currently playing sound" checked in "Volume control options".
All this can be accessed by right clicking the speaker icon in the System Tray.
No 3rd party program needed.
I'm using Win 7 (fully updated) and Avast free 7.0.1466
I know this an old thread and my first Avast post, but may help others searching for this.