2019 Macbook pro: Headphones stopped working after Monterey. The headphones work with iphone, and Magic Mouse works with Monterey. Tried everything I could find online to fix the headphones problem on the Macbook, with no luck.
Finally called Apple support when I got a chance, and the problem seems to be that Monterey includes a Bluetooth security update which is not supported by some (e.g. older or third party) Bluetooth devices. Thus, they will not work on a computer running Monterey.
It appears Apple is well aware of the potential for devices to no longer work after updating to Monterey but has not informed users of the potential Bluetooth incompatibility with some devices such as speakers and headphones.
Customer service was nice and helpful in terms of telling me the problem. But I've got nothing but expletives for Apple itself for not informing about this compatibility issue.
I asked customer service which headphones would work with Monterey and was told there *was* an article that would help me, but neither he nor his colleague could find it. All he could tell me is that I Monterey supports Bluetooth 4.1.