Same issue happened with mine - decided to follow the guide by @elmo41683 and found the exact same gunk blocking the holes in both of my earbuds. Gave them a good clean and the volume is crystal clear again.
Separated the cloth from the mesh.
Check out the gunk / grease in there! Seems like it could be a production issue since most of you guys reading this are experiencing the same problem.
Definitely not ear wax. Machine oil? Or maybe the sub-par glue that was supposed to hold the mesh? I don't know.