Ducks can lose their quacks for a few reasons, serious to minor.
Hardware disease - metal stuck in the throat (serious)
Mild virus - (minor)
Injury - something pressed against the neck (minor or serious)
Gapeworm - a parasite (serious)
Laryngitis from overuse - like people get after a sporting event (minor)
there are probably more possibilities
A good vet's help is great when you can get it.
I would:
examine the throat carefully,
listen to the breathing,
check the feed for moldiness
check the bedding for moldiness
check the feces looking for anything unusual
add apple cider vinegar to their drinking water (if they have a galvanized water bucket, I would switch to plastic or stainless steel), a tablespoon per gallon
My Carmella had laryngitis and I took her to the vet - no gapeworm. I tried ACV and it did not seem to help, so I tried a very small amount of colloidal silver in the water - several drops in a gallon of water - and her voice came back.
Einz's voice changed last year to a higher squeaky pitch and I think it's because she yelled so much at the new ducks.
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