Music distorted after upgrade to iOS 11.3

I have an iPhone 6 on iOS 11.3. After the 11.3 update, the music from the phone is dreadful. It sounds like those old record players stuck on a certain note... Triple-echoes, pauses, static. Really crappy. I checked it with and without headphones. The problem persists. No problem with the sound or music from Youtube for instance. The strange thing is it doesn't happen all the time. Sometimes it's without any problems. Other times, the problem goes on and on. The other day I got so frustrated, I took it to the Apple store... and ZERO symptoms! Sure enough, the problem came back in a couple of hours. It was like when you take your sick child to the doctor and the doc finds no symptoms. LOL.

What did I try?

1. Deleted all the music and re-synced. Problem persists.
2. With and without headphones. Problematic with both scenarios.
3. Hard re-boots. Problem persists
4. Music from other sources (Youtube, WhatsApp clips)... no problem.

What I figured was that it might be one of 4 things:

1. The album is messed up - unlikely... because it plays well on other devices and on my Mac
2. The sectors on the physical memory chip are corrupted? Unlikely
3. The Music app on the iPhone is buggy.
4. Electric interference from other household devices - very unlikely

Any help is appreciated.