Keyboard shortcuts are managed by browser, see: "about:addons".
In order to check your keymapping please open about:addons then press cog icon and select "Manage Extension Shortcuts"
By default they should be: Ctrl+Shift+1 : Left channel(mono) Ctrl+Shift+2 : Right channel(mono) Ctrl+Shift+3 : Stereo(default) Ctrl+Shift+4 : Mute audio
If for any reason you would like to keep old bindings then let me know I'll add feature flag to enable them.
