Hmm. It might be an emotional affair type situation...she's not "missing" is she like loooong shopping trips or working late when she is with an AP?
I wonder about mental health. If it was 50/50 chores and no screaming and is now patchy sleep, not doing usual daily activities, irritable/angry and less engagement with you (and DS?) there could very well be some mental ill health.
Playing music all night when she's sleepless is also odd and not consistent with an affair but with someone maybe trying to distract themselves.
If you haven't done already, I think offer her the chance to talk. Don't insist and you might have to ask more than once, something like
"Things are really tricky for our family just now. You seem to struggle with sleep and with feeling annoyed. Would you talk to me about it?" If yes, more gentle convo if no, say you'll check in again in a day or 2 - and do so, in the same gentle way.
If she's unwell, anything like "sort yourself out" while it may feel justified won't help.
I know PPs are saying about the double standard (if you were a women posters would be all saying cherche la famme) but the behaviour isn't standard cheating stuff.
Even if she turns out to be cheating there's no harm in offering support first in case it's her health. All you'd lose would be the "win" of identifying it.
It's very difficult for you OP and moreso when you're staying steady for your DS as well. Sending strength.