I made a post in the technical support area, but I wanted to share it here as well to see if anyone else is having this problem or something similar…
To start, I’m extremely disappointed that the BMW E34 M5 doesn’t backfire at all anymore! 
It used to have the best sounding & loudest backfires in the game! It also used to backfire a ton which was badarse!
The other audio Issue I noticed is sound/volume degradation. Right when you start the race (any car, any race, most noticeable in freeplay ) the sound is normal & sounds great, but after around roughly 1+mins the car sounds degrade & become muffled & very lackluster
I’m using the same exact setup & settings as I have since I bought the game when it first came out (digital standard version). I’m playing on an Xbox One S & have tried clearing cache, cold reset , hard reset , different Hdmi cables, different TVs etc etc with the same results.
Sometimes when you go off the track into the grass or sand/gravel the car sound gets back to normal/louder for a split second then back to the muffled, lack luster lameness.
My in game audio settings are basically everything at zero except the master volume. I’ve tried adjusting everything, prolly tried 100+ different settings!
Anyone else dealing with this?
UPDATE
Woot! E34 M5 is backfiring again 
they definitely aren’t as loud, but at least they are working again.
One thing I forgot to put in the original post was that the volume gets way louder when switched to driver/cockpit view, but then when you change back to a “normal” view the volume is wayyyyy more muffled & lackluster than before switching to driver/cockpit view…WTH!