I would keep this light. They may be the kind of couple who do not care, or be hugely embarrassed. I’d message via WhatsApp if you’ve been communicating that way:
‘hearing a bit of noise coming from downstairs, is everything ok down there?’
No mention of an argument, no embarrassment, no need at this stage to mention it is crazily loud what you are hearing. Of course if things escalate or continue after they have had read your message. (WhatsApp will show when a message is read) I’d call the police.