You don't say what source(s) are affected. I'm guessing TV or Cable-TV is the problem because music with vocals has voices and you say it sounds ok. Your speakers have no clue whether a sound is a voice or music so that lets THEM out and your receiver doesn't know or care either.
In the modern multichannel world it's usually assumed your electronics are multichannel. This receiver is old-school stereo.
The Center channel is usually where voices are found but some systems (like yours) don't have a Center speaker so they have to be configured to share the Center duty with the front left and Right speakers.This isn't possible or even normally necessary in your case. WYSIWYG
The audio coming in from 'wherever' should be correctly mixed to 2-channels at the source or you'll probably be missing something. I'd look higher up the audio food chain for where the voices are being attentuated. If a cable box is involved I'd look at its audio settings regarding stereo mix-down
Wait, I just had an inspiring thought. Centered voices being mono in nature would partially cancel out if your two speakers were out of 'phase' with each other. Bass would also suffer. Make sure your two speakers are wired identically regarding the red and black (or plus and minus) conductors. If turning the Balance control fully to one side makes bass of voices sound louder you probably have a phase mismatch. Reverse the wiring on either speaker to match the other. There is no absolute right or wrong for phasing as long as they're all the same.