Issue 7968: Added keybinds for increasing, decreasing, and muting audio.

This will behave differently from what the UI allows you to do. The UI doesn't allow you to ever get lower than 1% (thus never actually muting) while this allows you to get to 0%. It's a bit odd to have the UI state it's at 0% but it actually can't be set to 0% without the keybinding. Do we want to change the UI to allow 0% or do we set this minimum to 1%?