Ebay would be a great place to start. Also this site might have what you are looking for https://www.joesge.com/collections/sony-wh-1000xm4-repair-parts
Probably a burnt up component inside. your best bet is probably to contact the manufaterer and see if there is a warranty if there is not then you will probably need a new microwave. Some good places to pick one up for cheap would be fb marketplace or a thrift store or yard sale
If there is any way that you can contact the original owner you could have them remove it from their apple account or if you can convince apple that the device is legally yours then they might remove it. Whatever you do, definitely do not buy any programs that claim to remove it because none of them work