From Test-Scratch-Wiki
A sound on/off button may be useful when making a game that has music or sound that can be muted or stopped.
To make a sound on/off button, you need a sprite in the form of a switch. The switch must provide two costumes, one to represent when sound is on, and the other to represent when sound is off.
when gf clicked switch to costume [sound-on v] when this sprite clicked next costume if <(costume #) = [2]> then //assuming the second costume is "off" and the first "on" set volume to (0)% //if now off, turn off the volume else set volume to (100)% //if now on, turn on the volume end
This script can be modified to allow the volume to be changed.