bought house with Nutone 516 and ISB 53L intecoms; door speaker always audible but talk doesn't work right.

As above. I can always hear the outside. I've checked the wiring on all the intercoms--seems OK but there is an IC with a couple of other ICs daisy chained off it instead of in series, if that matters. The door chime works fine on all ICs. Anyway, I can always hear the street, and when I press the talk button one can not hear speech on the door speaker. When I press talk and "Door On" there is a loud buzzing. Any ideas?

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