We had downstairs neighbours like this once.
When we moved in they told us they were glad we had no children as the people before us had two and they were awful. I think they called them noisy little shits, and they were very happy to see us. For all of five minutes.
Then they started to complain about us. We were walking too loudly (all floors carpeted and had underlay as well), they could hear our (very small) dog walking about, our washing machine was too loud, they thought we were putting rice down the drain and blocking theirs (turned out to be a crisp packet their daughter dropped in their garden), our toilet flushed too loudly, they could hear us taking a bath, they could hear our TV, our telephone rang too loudly and they could hear us talking on it.
Worst and most bizarre, they could hear our fish swimming in the fish tank and it was keeping them awake.
They never said anything about the fact that we could hear them having screaming rows every day, or slamming doors, or starting a motorbike under our window at 5:30am, or the fact that their cat would piss on our doormat every day and attack us if it was on our porch and we came out (we had one of those flats with outside stairs and a porch at the top). They ran their washing machine whenever they liked and had loud music and TV at all hours of the day and night.
At first we did our best. We made sure we didn't walk about inside with our shoes on, we didn't use the washing machine when they were home if we could avoid it, or we made sure we only used it in the day time and not too early in the morning or late at night.
We made sure nothing was put down the sink, we did our best not to flush the toilet late at night, we didn't have baths late at night, we tried to keep the TV down low and changed the ring volume on the phone.
We explained to them that I have hearing difficulties but now we knew things were too loud for them we would change the volumes (until the day they complained the TV was too loud and it wasn't actually on because I was home alone, ill and asleep when they knocked to complain, then we knew they were just taking the piss).
There wasn't much we could do to stop the dog walking or the fish from swimming, but we had a word and asked them to walk and swim quietly.