You need to go open the song in a program called Audacity, look at the the grey box that comes up when inserting a song. There should be an arrow, click it and select "Split Stereo Track". After you do that will have two separate tracks, click the arrow on each of them and select "Mono". Then you are done, go to File > Export > OGG.
Things to consider
1. Don't add any spaces in the music file, use _ it will show up as a space in Blockland
2. The file can't be more than 990 or 980KB. (Make the song shorter or lower the Hz quality at the bottom bar in Audacity)
Good luck.