monitoring the alternator current to detect the speed

I don't think this could work.

All it could do is adjust volume proportional to engine RPM, not vehicle speed. So what happens when you go up through the gears? It gets louder, then quieter, then louder, then quieter, etc.?

And since the alternator output, once you reach threshold RPM (just above idle for most cars) remains a function of electrical load required by electrical accessories currently in operation, I am not sure just what the JVC would have been monitoring.

Might be interesting to find out what was really going on with that unit.

tanstaafl.