It would have been nice if you mentioned the sound as what you were more concerned and a suggestion could have been given.
The free (for Home and non-commercial use) version of avast has an interactive requirement, the Pro version has more flexibility and you can set it up to take actions automatically when an infection is detected.
What version of avast do you have ?
You can disable avast sounds, but that disables all avast sounds, Program Settings, Sounds.
There are other options which are dependant on the version of avast you use.