To set your Sounds go to the menu above in BlitzIn and click on Edit. From the list choose 'Settings' and now you can click on the Sounds tab.
In the top right corner you find 'Ring bell?' Here you can decide if you want to have sound on no moves, on any moves or only your opponents move. (You can also use the variable 'bell' for this. "help bell")
Or: Use the keyboard combination Ctrl + S
This brings up your Settings window. Click on the Sounds tab.