Sound Issue With iTunes: music voices are faint

I'm having a bizarre problem with my Mac, whereby the music I play sounds great but the singing voices are very faint, like they're in a back room somewhere -- backup singers I can hear well, tho. I've tried everything I can think of -- which may not have helped much, like playing with the MIDI adjuster, running Boom, trying the built-in mixer, nada.

I've read of similar issues that Windows users have with iTunes, but I'm on MacOS High Sierra.

What can I do to fix this mess?

Thanks.