Simrad VHF radiotelephone handset speaker - poor sound

It may not help, but my experience with earpieces (if it's not the cable to the blackbox itself), is that moisture collects grime, and that
can cause reduced volume. I would try to clean the earpiece before trying to replace it.
Someone here might come-up with a replacement supplier for you, but you asked for 'any advice', and that's what I'd try first.
Good luck.

 

Check if the handset/microphone connector in the VHF radio black box is similar to other transceivers and test/borrow another. You must isolate diagnosing to the main unit or the handset. There is nothing simpler than connecting a speaker and listening to its audio. Can you determine which pin is audio out in the 'black box' ?
Check the rear panel for an external speaker jack and confirm bad or good audio by hooking/plugging a speaker there.
There should be no feedback as it works simplex PTT. Do you have the unit operator's manual ?

?----> http://www.2040-parts.com/simrad-rs81-marine-vhf-radio-handset-only-ahs81-i2305324/
?----> https://www.manualslib.com/manual/1288907/Simrad-Rs81.html?page=32#manual
?----> https://www.manualslib.com/manual/1288907/Simrad-Rs81.html?page=33#manual

 

I would open the tinny distorted sounding handset and connect another similar speaker instead of the original to diagnose if the speaker is bad; then replace it with whatever performs in it. If the audio amplifier or path in it is producing the same problem with the replaced speaker, take some pictures and come back with observations.