help i cant use a mixer with microfhone and musica at same time

So I'm trying to connect my mixer to the computer through a USB cable and then through my mixer I would like to use the microphone and to put some low music, i have tried sharing sound or through the mixer,   but I don't know why in both ways, the voice of the microphone and the sound of the music they compete each other in the zoom so the boys disappear for periods of time, loke muted.  so I want to understand why in how can I solve this problem thank you!