Tea tree oil is an ototoxin. I had what I thought was a wart on the backside of one of my ears, and when I put a cotton ball with one drop of tea tree oil on it, I immediately heard ringing and ripped it off. The ringing has remained. It's not severe, but I've read stories online where people have put tea tree oil directly in their ear canal and were badly affected. This has to be relatively common, with how popular tea tree oil is in products that you put on your head. I bet people don't realize their tinnitus is caused by the tea tree oil and continue using the products.

Also, I have a friend who has severe tinnitus as a result of an ear infection. He really struggled with it in the beginning, but hasn't mentioned it in a while.