Sorry, as I worked in research with these I assumed they were well-known terms, but some info: https://www.esense.io/

http://omnibuds.tech/ (Both from Nokia Bell Labs)

And a literature review from the designer of the OpenEarables: https://dl.acm.org/doi/abs/10.1145/3550314

Eating detection might be one application:

https://auracleproject.files.wordpress.com/2018/08/bi-ubicom...

https://auracle-project.org/

On thing that I've read about is ear based EEG. Easier simpler contact for things like sleep studies. They don't list that as supported, but it is an example.

https://en.wikipedia.org/wiki/Ear-EEG

One of them is authentication, or at least in principle. Seems somehow much less dystopian than the eye thing Altman's doing, but I can't exactly put my finger on why that is...

Though they never explicitly confirmed, I think some of this of tech was what enabled the dynamic and personalised equalisation adjustment in Nura headphones.