video with background music

@MagicMax 

Is the music in the video being picked up by the microphone?
If it is being picked up by the microphone, the Zoom's noise canceling feature will kick in and mute the music.
There are two ways to counteract this problem.
1. select Original Sound for Musicians in the audio settings of the Zoom app.

2. distribute the video in Screen Sharing mode. If so, be sure to check Share sound.