If it's a fight, they're stressed.

If it's a domestic dispute, they could be more than stressed. Someone's life could be in danger. Calling the police is a good idea. If you can do so without stretching the truth too far, tell them you believe they're about to get violent with one another and you're concerned that someone is going to get seriously hurt. Since 911 calls are recorded, police departments try to avoid the bad publicity that comes when a reported domestic dispute gets ignored until it turns into a murder investigation.

Within about 5 minutes, the cops walked into the party, confiscated the stereo and dispersed the crowd.

I like this approach and wish more cops would do it. Keep the equipment until they pay a stiff fine. If they don't pay the fine within 30 days, sell the equipment to help defer the costs of multiple police visits to the home.

When I was stationed in Germany, over 80% of the guys in my barracks was a shift worker. Anytime of the day or night, some people were trying to sleep. One thing just about everyone did back then was to buy a stereo system so naturally people wanted to play their music. Our unit had a simple policy. The first time someone complained about your music being too loud, they were warned. The second time it happened, the equipment was confiscated until they transferred out of the unit (which could be up to 3 years away). Not many people needed a second warning. Most of us bought a good set of headphones so we could play our music even when our roommates were asleep.