You did a darn good translation, ottery. I suppose we could offer tweaks forever, and never come up with a perfect translation. Nonetheless, here are my tweak submissions:

Mit ihm zu concertiren;   
To concert along with him;

How about, "To croak in concert with him"?

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Ein jeder nahm das Maul recht voll,
Each one filled up his mouth,

"Each one brought his mouth in fully"?

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And I have to say that the frogs outside my window here in the Sierra Foothills have been driving me crazy this Spring. They're just little things, but they sure make a lot of noise, dammit.