Same thing happened to my Nexus 5x, i'm still trying to find a solution but nothing yet.
I think this is an LG problem because my Nexus 5 also died after 2 years the same way...
The thing is that I used to connect my Nexus devices with the aux connection in my car. A few days ago the volume was lower and don't know what could be, I've tried wit my galaxy s3 and sound is loud and perfect but not with the Nexus.