Tinny speakers internal and external

I've owned my 2012 Macbook Pro for a long time now and have always thought the speakers were pretty good for a thin laptop, but something changed the other day. I opened my laptop and started playing music and everything sounded tinny and full of treble with no mids or bass. I use these speakers a lot and immediately noticed the difference. I plugged in my studio monitor speakers thinking the speakers had failed, but was surprised to find the exact same issue through external speakers. I tested the external speakers with my phone and they were fine, so it's definitely something with my macbook.

I believe I have tried all the normal resolutions through the sound settings and audio midi setup. Does anyone have any suggestions on what could have happened? I actually decided to update to El Capitan, hoping this would update something, but no, still the same problem.

Thanks in advance for any ideas! (It's driving me nuts!)