Sound files are .wav files. (Or vice versa, there are a few other formats..mp3etc)
The one that plays the update sound (by default, it can be changed) is vpsupd.wav. Right click it and see if "Record " is an option. If not, no point in continuing. If so, and you're up for it, right click it, select "copy", navigate to C:\Windows\Media, right click in a blank area of the folder, select "paste".
This copies the sound file to the Media folder, where all the system sounds are.
From the Avast icon, select "Program settings", then "sounds" and click the settings tab. This opens the sounds configuration window. Click the "sounds" tab, scroll down to Avast antivirus, select the sound you want to change (Automatic VPS update) and browse for it, where you just copied it to. Once found, select (right click) "record" and then do what you want with it. Good luck.