For all the Ballroom Guide music, I followed this pattern.
1) Play the song in the background
2) Use this website to find the BPM of the song.
3) Load the song into Audacity, select Effect and then Change Tempo
4) Enter the current tempo of the song, and then enter what you want the new tempo to be.
Voilà
P.S. You can also change the speed of the music, but that makes it higher pitch if you speed it up or lower pitch if you slow it down. Using the change tempo function seems to put micro breaks doesn't affect pitch, but if you slow it down too much the song gets choppy. In order to make a large change, like from 135BPM to 90BPM I had to use a combination of speed and temp, which did result in the song being lower pitched.
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