What do you do when you hear a domestic dispute?

Last night at 4am I was awoken by some of my neighbors (sounded like a couple) really getting into it. I don’t know for certain if it got physical or not, but at one point I think that I may have heard the woman get slapped, and I definitely heard her sobbing “don’t hit me” near the end of the argument. Needless to say, this was really upsetting. What is the right course of action in these circumstances? I was really freaked out but didn’t call the police since I wasn’t sure if what I was hearing was violence or not, and I live in an area where the police are stretched pretty thin. Now I feel really guilty about not calling, and want to have a plan in case it happens again.

What would you do?