My turntable sounds like the speed is too slow

If the speed is consistent and correct then focus needs to shift to the cartridge setup.  Has it always had this “muffled vocals” presentation or is this a recent change?  Check VTF, and alignment of the cartridge as your next steps.

Well, if the speed is correct:

it's more than likely coming from the cartridge"

how are your alignment skills/have needed tool set?

alignment: was it working well before? were/are the two screws tight (try moving the cartridge body by hand, twist sideways, move front to back, IOW, IF it was not tight, it could have moved.

arm height, parallel to lp when playing?

stylus worn/perhaps loose from cantilever or rarely chipped; cantilever flattened (low rider), or bent sideways; tracking force or anti-skate or both seriously off

Dear @alexnguyensound  : A gentleman mentioned re-start the cartridge/tonearm overall set up and additional you need to make sure that either LP surface and cartridge stylus typ are well cleaned.

Maybe the last source could be a " problem " in the tonearm bearing that does not flow " freely ".

Regards and enjoy the MUSIC NOTDISTORTIONS,

R.

Allot of great advice already so the only thing left is what the TT is sitting on or next to may be contributing to you problem?

Playing a 45RPM record at 33 1/3?