I've had no sound issues whatsoever with my 2017 iMac. It sounds fine, really nice in fact, and not "muffled" at all. I think those who feel they have muffled sound should get it checked out. Also, I don't have a 2015 to compare mine to - but to all of you who are saying the 2017 sounds worse, it's really hard to know if there's any truth to your statements when we don't know what/how you are comparing. Are you comparing the exact same source material with the same software and same settings? It could be that you had Sound Enhancer or EQ enabled in iTunes on your 2015 and disabled on your 2017, or something along those lines.