Batboard

I have a UHF CM200 that the power up beep and a received signal with or without modulation sounds like a duck call. Or maybe a fog horn. Have you repaired one like this?

Since you stated even the power up beep is distorted than we know its NOT in the reciever stage and being introduced in the IF stages of the reciever stages . This can cause what you described but the power up beep eliminates this.
Probably just a crappy speaker but they usually sound "tinny" unless its mounted face up and you have a ton of crud that settles in the cone and loads it down .
The audio amp usually works or it does not.
A power source ( vehicle battery or external supply used in a base station application) can also introduced noise if the filter caps in the source go bad. The radio also has filter caps as well that could do this , but they are generally pretty stable and reliable.
You need to look at the input to the audio amp as well as the output with an oscope to quickly determine if its coming into or leaving the audio amp .

Funny, as I myself use the term Donald Duck audio when the IF freq's in the reciever go astray , but since its in the power up beep that's eliminated .

Probably just a speaker that has been used hard and put up wet

Agreed. Start with the speaker, and troubleshoot from there.

Not the speaker or power supply.