Thanks for posting that video.
The various sounds and relatively "good"
fidelity seem much better than what a piezo
transducer could produce, IMHO. I think its
more likely to be a dynamic (voice coil) speaker.
Have you had the chance to open up the
receiver unit, and take a look?
Post a pic?
Dynamic speakers can break, if grossly overdriven
(or poorly made). But I still suspect the circuitry.
If your comfortable working on a live unit, opened up,
then plug your receiver via extension cord, and examine
the speaker. Could try wiring in a substitute known-good
speaker, to see if the sound quality improves.
Next, carefully, with a DVM, probe the power supply
DC voltage. See if the DC reading drops by, say, over
20%, when the unit is emitting a (loud) sound.
Repeat, with DVM set to AC volts; the AC reading
should be minimal or zero, even when producing a
sound. If the reading jumps to, several volts, then
either a rectifier diode or the filter capacitor is bad.
= RS
check