Answers

Spray insulation foam close cell.

The least expensive option is going to be to create white noise.

Otherwise, you'll need some type of sound insulation and dense pack cellulose might be able to be retrofitted into the wall cavity. But determining your level of performance expectation is the hard part. How perfect do you expect it to be? Is this a common wall to another unit, or all enclosed within yours? It makes a difference.

It’s most likely a common wall. I’m extremely worried about neighbors hearing extremely private noises.