I saw/heard one last July. It was in a Sopwith Pup. It takes a pretty good size model to implement. Really was fun to hear the engine start up, rev up and fly by. This particular setup was not quite loud enough, it did not compete to well with the ambiant noise that day. On a quiet day it would have been great. Gas motor sounds dround it out.
This wasn't the plane but it was a lot like this.