Not sure which avast version you are speaking about if the latest version 19.8.4793.544, then try this:
Avast UI > Menu > Settings > Notifications > uncheck the Enable Avast Sounds.
There is another option to 'Use Silent Mode and turn off all pop-ups, alerts and messages', but I feel this is a step to far as you don't know what is going on. Any alerts would be dealt with automatically depending on user/default settings and you/they may not want that.