I also have a Ball vario (702) with the battery(s) inside. I like it's response and the back-up nature of having a fully self-contained unit. No audio, but I would like to get a schematic for the audio portion. I pulled the unit apart and looked at the PC board briefly. It looks like the unit uses an output from an OpAmp, probably a voltage, and creates the audio tone from that.

Does anyone have details they can share/send?

Mike