About the volume part of the question/answers, I think that the suggestions are only “workarounds”, but not solutions.
I mean, the “answer” seem to be to completely avoid the voice, using one of several methods. All those methods have their “cons” (which, IMHO, are far more important than the “pro” in this case).
The question, aside from setting it to male/female, is about “volume”. The “evident” workaround is to lower the volume of the system (not only for “Avast voice”) :P, but my guess is that this is not the real answer the OP was looking for.
Unfortunately, I don’t know the real answer. If someone knows how to change the volume of (only) Avast notifications, I would also like to know it.