Sorry to necropost to this thread, but for anyone wondering, the Bisson muffler that suits the OS 75 AX is only a exhaust redirector.
We acoustically tested it at my flying field with a hand held digital measuring device and at the exhaust size at 7.5m we got readings of 97db. This is above the allowable limit of 93db at our field.
There are no baffles inside other than at the throat. If you need quiet, this is not the muffler for you.