If you were left wondering after my Bang cow pen post, the answer is a tape recorded loop.
Or, at least, that’s what I surmised when getting this picture of a street vendor’s bicycle.
The loudspeaker to the left of the front wheel is hooked up to an amplifier on the handle bars which is powered, via some dodgy wiring and a fuse, from a battery located out of shot on the back wheel.
I didn’t have enough Vietnamese to get a proper explanation but sign language seems to suggest it is a audio cassette loop. So now we know.