According to the service manual copy I have, there is no mike gain adjustment on the TM-281. There is a maximum deviation level adjustment, which is three possible settings for low, medium and high. You need to get the rig into the "adjustment mode" by shorting a couple of terminals on an internal circuit board while powering the rig on.
But deviation and mike gain are two different things.
First thing I'd recommend is to assure your buddy is "close talking" the microphone by holding it right up against his lips and not farther away than that. The mikes are all somewhat noise-canceling and need to be "close talked." Using one from a few inches away works poorly with all of them that I've tried. Is he doing that already?
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