Unfortunately Chess.com does not have a volume slider that let's you turn it up or down, the only way to do this would be to go into your volume mixer and turn down the sound on whatever browser you're using.
This is just outrageous... Chess.com is a barely usable website honestly. Pls just copy whatever [another site-- MS] is doing with their UI so we have more sites to play on, it's not that hard
how to adjust sound, not just turn it off/on, default setting is too loud for simultaneous itunes playback