In addition to Shackman's invaluable info, on a practical level I found that swapping the multiple AA's for a 9v set up also decreased the volume slightly, meaning that you have to turn it up to get the same results, which then increases the drain on an already less effective battery.
To get back to the subject matter, I mounted my speaker under my right shoulder bell. No one (except kids and ewoks) can see it, but it's as loud as you like, and sufficiently close to the bucket that the voice appears to be coming from the right place, and there's no feedback.
Si