Yes, and it's quite easy!
Locate the three icons in the bar at the top of the app, showing a volume icon and people singing. That's where everything happens.
Whenever you play a song, volume slider will appear when you hover those icons, allowing you to change the volume of the backing vocals, and the lead vocals.
It also works with most duets in our catalog, a feature that's exclusive to KaraFun!
And if you are wanting to set default vocals (lead and background), go to the menu bar > KaraFun > Preferences > Playback.