Onan calls it the resonator, not a muffler.
It is meant to soften the irritating pitch and the sharp punches of the firing cycle.
They are non-committal in reducing the volume.
... can reduce sound levels in
some installations up to one dB(A).
It is an air-cooled engine; there's not much you can do -- The noise emanates to all directions.
ps. I wonder if anyone has tried to put Fatmat on the Onan.