iPhone speaker sounding terrible all of a sudden

I have an iPhone 7Plus, and for some reason the sound coming out of it has turned to garbage. Whenever I play music through the speakers, the audio coming from the earpiece speaker (right channel of stereo, I believe) sounds terrible and muffled. Which is totally ruining the sound of anything being listened to. If I cover up the earpiece the remaining channel coming from the bottom speaker sounds fine but it's only half of the mix.

I'm not sure what could have happened to suddenly make this sound terrible. I know it wasn't an issue before. The phone hasn't gone through any trauma, hasn't gotten wet or anything.

Any help or suggestions would be greatly appreciated.