Repair faulty 35mm audio jack

I assume you’ve tried different headphones? It definitely sounds like a loose connection. You can try cleaning the port out with compressed air and a tooth pick with some rubbing alcohol(dipped on the tooth pick)

if it’s not due to dust,it’s probably a bad port. It would require a replacement. This means disassembling the phone. Sometimes it is soldered on the board,other times it is a separate part that plugs into the board.

You can also get Bluetooth earbuds if the above is too much of a hassle.

I had this problem on a Nokia 6.1. I found the plug was not pushing in all the way. After a careful look I realised the jack socket was a closed space and sand or dirt had somehow accumulated in it, stopping the plug entering properly. After switching the phone off for electrical safety I gently scraped around the bottom of the socket with a fine pin, loosening debris which then shook out when it was turned upside down. Powered back on and all had returned to normal.

This may not be the answer for you but it is quick and easy and a simple fix if it works.