Why do I only hear a sample although I've bought this track?

In order to reduce the page loading time, you will only hear a short preview of each multi-track when you modify it (even for the tracks you've purchased). The complete version is available upon download.

If you want to get each track separately, you just need to create them separately by muting all the other tracks and downloading the file for each separate track.

You can modify your Custom Backing Tracks as much as you want for free.