The OnePlus 6 got upstream audio support a while ago, but the OnePlus 6T has no audio support for now.
Since Linux 5.19 (the development kernel, used in postmarketOS and Mobian) OnePlus 6T will also gain support for audio playback. It's not the final upstream Linux driver, but it's a step in getting to a fully functional phone.
The driver needs official firmware, available here, but I assume at the time of the 5.19 release it will be packaged by both Mobian and postmarketOS.
While looking for an upstream driver repository, I noticed that there was an officially looking repository, so I wrote a message to Goodix about considering upstreaming their driver officially. Sadly, they decided for now to not spend resources on upstreaming.