I've always been extremely chary of the once popular effort to "intervene" with people, or gang up. Plus if she has run on stress for years, being called on it could be a "snapper". So, no advice from here (now I say that, after saying earlier that people should react - I was off on that if everyone spoke up, she might.. what?).
It's really her and the husband's business, her psychological health, and you just abutted with it. But, dayum, the baby...
Was it punkey that mentioned stress? Once I start to understand that, then the loudness seems more iffy. On the other hand, maybe she's hard of hearing?
I'm out of my league, but it's a sad situation.