I'm having problems with my audio

I’ve been looking all over the web for a solution to my problem, with no luck. I’m using a macbook air that is a few years old. Operating system: X Yosemite. Mic: Shure P27 USB. Its a direct plug mic which allows me to hear my voice. I had it working about 6 months ago so i know nothing is broken. I’ve been through a lot of settings and all my essentials are correct as far as I know. When I’m all connected and ready to record I upload my instrumental (Beat), to this point everything is fine; I can hear my voice and background noise just fine. When the instrumental starts playing I can barley hear it…barley. i’ve tried everything, spent 6 hours on the web and trying to fix it. Last time i clicked a few things and it fixed itself. It barley…barley plays the instrumental through my headphones, its fine through the speakers. Any help will be greatly sweeeeeettttt. thank you.

Shure PGA27 USB by any chance? We’re doing this blind and across multiple time zones, so try not to glide over errors like that. It takes us time to straighten out what you really meant.

I can’t find P27-USB.