1) I have neighbours who scream at each other all the time, they’re in their 80s and have probably been married for 40+ years. Point being: there is a possibility they are just the type of people who scream, not good but not necessarily violent. Consider that the Rov may have more knowledge about the case which he isn’t at liberty to reveal.
2) Re the issur moiser. I don’t know the halochos but have heard (in a different situation) it can be OK to call the police without saying the exact address. E.g. if it was from house no. 48 on the road, tell them it was 46-52, you were passing by and couldn’t tell exactly which. If/when the police come they will figure out for themselves, so you weren’t technically moser them. OBVIOUSLY you must speak to a Rov before applying this!
3) Perhaps try a different Rov, say you’re concerned about how the first Rov is handling it. Hopefully Rov 2 will talk it over with Rov 1 and you can have yishuv hadaas that it has been handled well.
4) Someone said record it, not sure if its OK to record another couple’s arguments, perhaps it would be more acceptable to phone the Rov while they’re at it if he’ll be able to hear that way. If you’re really worried maybe you could consider hiring a private investigator. (New thread on YWN: private investigator recommendations…). But I don’t think its OK to start trying to find proof yourself