Maybe itís my toy box system/setup but wonder if anyone has good advice for this?

If I want to isolate just one layer of the track eg vocals or bassline only. I canít really get that effect properly. It still plays the rest of the track but a bit quieter.

Is it my files (mp3 off internet ssssh)
Mixer - Numark mix track pro 2
Software - serato DJ pro

I am looking to upgrade my mixer soon. Just started getting a few paid gigs so about time I got something more professsional. So if this is the issue then recommendations of what to go for next would be great too. Not too pricey though