For me it is certain singers that apparently everyone else understands but I cannot without knowing the words ahead of time. Not just mumbling, some voices just don’t register clearly for me if I don’t know what they are saying.
It can depend on the mixing, too. Not just in regards to volume, but also in how the vocals are edited. My recent obsession has been Dusk at Cubist Castle, shit’s absolutely amazing. The way a lot of the vocals are mixed and processed are super cool, like layering the same lines over themselves five times over with subtle delays and panning, it sounds real cool! But it makes it sound a lot more distant to me as a result.