Are these background voices/vocals singing to the tracks?

The short answer is: Yes and no.

The karaoke tracks are meant for pure karaoke purposes and as such, there are no voices singing along to the music. It defeats the purpose of karaoke singing if there are vocals singing along with the music!

However, the tracks are professional recorded such that, for selected songs, there will be audible background vocals during certain sections of the music, (especially during the chorus).This is adds a sense of realism as it follows the original songs where backup singers enhances the main vocals during chorus sections.

It also provides you with a "live" singing environment so much so that you'd think you're actually singing with a real band.