How to Play Music while Recording Video on iPhone
Step 1 — Start music playing
Step 2 — Go to your camera, not video mode but camera mode.
Step 3— Optional. Tap the arrow at the top and select aspect ratio 16:9 so the video is recording in wide screen instead of a cropped photo mode.
Step 4— Tap and hold the take photo button to start recording video.
Step 5— Optional. If you want to continue taking a video, swipe your held tap to the right.
Learned this trick from this YouTube video — https://www.youtube.com/watch?v=xDpjnozl8PE