When you add a video file to the timeline then, by default, the audio of that file appears in the Audio 1 track. You can add a music file directly to Audio 2 track of the timeline. Then you can Video Edit Magic's volume tracks to control the relative volume of the two audio tracks.

The Volume Tracks of Video Edit Magic supports volume changes from 0% to 400% of the originally authored volume.

You may find our article on useful audio effects using the volume graphs useful:
https://www.deskshare.com/Resources/articles/vem_UsefulAudioEffects_withVolumeGraphs.aspx