Can I add a crossfade between media in the editor timeline?

Yes, you can create a crossfade using the fade-in and fade-out controls under Volume & effects by adjusting the volume levels of two overlapping music segments. 

1. Zoom in on the timeline.
2. Drag one music segment slightly over the other so they overlap in the timeline.
   This overlap will be the length of your crossfade.
3. Hover over the first segment in the timeline and click ••• [the three dots]. 
   
   • Click Volume & effects to adjust the volume settings.
     • Drag the controls to set a fade-out effect at its end.
4. Hover over the second segment in the timeline and click ••• [the three dots].
   • Click Volume & effects to adjust the volume settings.
     • Drag the controls to set a a fade-in effect at the beginning.