I did some experiments on the volume control system. Two volume controls for separate control over the speech and beeps is a bit unsatisfying, so I've changed it to instead be a balance control followed by an overall system volume.

While I was at it, I discovered that cutting the preamp gain in half made for better volume control range and less noise. I'm pretty happy with the result.

The only question left really is whether to make the volume control external or not. It's definitely more convenient to have a knob, but that adds to the cost compared to a board mounted trimmer.