That's probably just your typical hardware failure - speaker failing.Hi All, newbie here. My left speaker is muffled on a Mid 2015 MBP, Retina, 15 incn. I just installed Mojave v 10.14, earlier this week. Headphones sound fine. I used Audio MIDI setup to mute the left speaker, the right sounds fine.
I live in Guam, so no Apple store. Anyway, just want to report my issue for the record!