This is probably the most-requested feature so I'll give it a go. I initially didn't consider it because you're supposed to play it solo and use your device's overall volume control if desired, but I can see the appeal. However I really don't want to clutter the interface, so I might hide it behind a settings icon.

Thank you for making this, it's fantastic, please don't clutter the visual.

A mono button would be nice but I can usually use accessibilty for that.

Keyboard control?

I've added a volume control. It's hidden behind the settings icon at the bottom.