Hi Lucia
I don’t have a solution for you other than having your owner convince the OC to add whatever insulation is possible.
However, I am posting to let you know I purchased a unit in a lovely beachside location with only 4 units & after living in it, found out that I can hear not only the toilet flushing in the unit next door but every tap they turn on, them cooking, talking etc. I can also hear the next door neighbour’s (house) shower & when they walk around their house. I can also hear the clinking of spoons in cereal bowls from another neighbour whose kitchen in on the 2nd storey so the sound wafts down.
AND something about the location is so quiet that I can hear people walking in the street (I’m talking about hearing the flip-flop sound of their thongs!) – my street leads to the beach so there are heaps of walkers. I can also hear people whom congregate in the car park at the Beach (I am 3 houses away). I can also hear parties in the Surf Club even though that is way at the other end of the Beach. It’s incredible & I have never experienced it before BUT I can also hear the sound of the waves so I’ve learnt to accept it & learn to live with it – not easy but I choose not to move & I could insulate the common wall between my unit & next door as I own that one too but it would compromise space. At least I have control over whom lives next to me & we are single level (ie nobody above or below).
Doesn’t solve your problem but I hope it makes you feel better in some small way. Gotta love old buildings (sometimes not!).