Misophonia/Chewing sounds upsetting teen

My son had this problem starting in his junior year of high school.  The noises his dad made while eating were a particular irritant. He would get absolutely furious, and my husband did not handle it too well.  Some research I did online seemed to show there were therapists who could treat this, but my son left for college before I found anyone.    When my son was home from college over winter break, it did not seem to be as much of a problem.  I also noticed that my son is smoking more pot, and that may be related to the improvement, although I really do not recommend that as a treatment!!  It does make me wonder if misophonia is related to anxiety. Honestly, if the headphones help your daughter, I would just let her use the headphones.  Be warned, too, once my son pointed out all the "smacking" noises my husband makes, they started to annoy me too, although I usually can keep quiet about it. 

I have misophonia, as do all four of my siblings. Eating sounds are unbearable to all of us, though I think mine has gotten maybe 50% better through meditation. None of my sibs are interested in trying that so I can only speak to my experience. For the most part we just make sure we take earplugs with us EVERYWHERE. Because you never know when someone will open up a bag of snacks.

Another alternative is to get some white noise onto her phone (either through a meditation app or iTunes) and then set it to the sound of a running stream, the ocean, or whatever, and put on headphones. That works really well because you can turn it up as loudly as you need it to be—essential for worst-case scenarios like a gum chewer on the next set on an airplane. Also, it makes the misophonia invisible to others because it just looks like you're listening to music.

It sounds like you understand your daughter is not just imagining this, which is great, because it feels horrible to be all alone with it. 

My son struggled with the same thing for a couple years when he was about 13, 14. We bought him the most soundproof headphones we could find on amazon and kept them on a hook in his room. He would get instantly angry at the sound of chewing and it would cause quite a bit of stress for all of us. He's now 15 and we realized a few months ago that he never uses the headphones anymore. He has always been noise sensitive- his teachers said he could hear every little sound the kids made and it made it hard for him to concentrate in class. But little by little he seems to be outgrowing it. I remember going through a period of chewing noise sensitivity when I was a young teenager and even today my husband gets bothered by the sound of me eating nearby and vice versa. I think my son (and to a much lesser extent my daughter too) will always be bothered but he doesn't have the explosive reactions anymore.