Well, it's simple. Get Audacity, and load the MP3 or whatever you want to make into a song. Then, drag the mouse over the area you want to loop (this is how you cut the song). Then, at the left part of the stereo stack, you will see this dropdown menu at the side. Click that then click, "Split Stereo Stack". Then, just click on the dropdown menu on each track and click "Mono". Then just go into file, then click "Export as .OGG VORBIS File" Or something like that. It will tell you you will need to mix it to be a successful mono file, so just hit OK and it will do it automatically. Save it with no spaces, but if you want a space use an underscore. Then when you try it out in blockland, it should work. :D
Hope that helps.
If you need further help, check this tutorial:
http://forum.blockland.us/index.php?topic=88838.0