I have a child who won't wear a seatbelt.
As a pp said, there are other solutions. There are several harnesses available, in different styles for different needs. Not ideal (they aren't recommended, in case of interfering with the seatbelt working properly) but that's a small compromise compared with an unrestrained passenger. Doesn't stop the kicking, though, or grabbing and hitting.
There isn't an alternative to headphones, is there? Well, there is - a speaker. The thing which is being objected to.
Incidentally, my child won't wear headphones because she was forced to wear ear defenders at her preschool. She hated them, but was unable to speak out against them, and the staff there noted that if they made her wear them, she was allegedly calmer (umm, no, what she was was withdrawn and isolated, terrified, and not able to speak out because she knew she wouldn't be listened to).
9 years later, she is able to tolerate headphones for about a minute. Because yes, I have tried, and worked endlessly to try to make her needs at least partially socially acceptable. She has worked so hard, and yet the payoff is pitiful, and not really of any use. Sums up a lot of her life, tbh, and yet she still doesn't give up and keeps trying.