Soundproofing air conditioning unit
Called a furr down unit, where the blower is in the ceiling. Used in condos and apts. Very common.
Not certain what can be done as far as soundproofing but the blower may be able to be lowered in speed. This would quiet the system down IF that blower motor can indeed be slowed down.
In residential applications where the return air grill is located close to the blower we can (and have) extend the duct, loop it around in the attic for quietness. So although the duct run from the blower to the return only needs to be let's say 6' we can run it 15'-20', 10' out and away from the return and then 10' back and then connected to the return, where the filter is. This dampens the noise greatly. The longer the duct, the less noise. In your situation that is out of the question.
And I don't know of anything that can be done as far as literal soundproofing.