Music played from Cloud becomes muffled after a while.
In Songs in the Music app will suddenly soun… - Apple Community
the OP reported that music will suddenly sound muffled, losing clarity. They reported that this was happening in the Music app. I am here to add that it also happens in Spotify. It appears that some sort of throttling is going on at a system level.
Anyway, the OP also stated that they were working with Apple support and running some sort of special diagnostic software. However, no resolution was ever posted, so IDK where to go from here.