It seems ever since I upgraded my iPhone 7 Plus to iOS 11.3 I've been experiencing this strange audio issue playing music sometimes. I'll be playing a song and then when I lock the screen the audio starts to sound choppy, or it sounds as if it's trying to scrub or fast forward in a way. I have this old system activity app on my phone and there seems to be plenty of usable memory. CPU for both cores in the GUI shows one core at 20% usage, and the other core fluctuates to 5-10%. (If anyone has a better activity monitoring app let me know.) This never happened prior to upgrading as well. Seeing if anyone's had this same issue out there. Going to try using Bluetooth headphones instead of the included earphones with lightning connector for now.