Mute is typically done through a dedicated key on some media keyboards, or through F6 or F7 (when used as media keys) on many laptops and some desktop keyboards. All it takes to trigger a mute by accident is Fn+F6 (or F7) and often Fn+F6 (or F7) will toggle it back off.
This is another of those instances where make and model information as well as whether any media keys function is enabled (and for a screen reader user I'd very, very strongly suspect it's not unless the machine is brand new and untweaked yet). Some makers do highly unconventional things as far as what function keys correspond to what media functions.
Gene's advice about using sndvol and getting a cheap external USB soundcard are both things to try and do.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
The depths of denial one can be pushed to by outside forces of disapproval can make you not even recognize yourself to yourself.
~ Brian Vogel