I found this on how to edit music for your gymnastics floor music on wikiHow:
How to Edit Music for a Gymnastics Floor Routine
Tired of hearing your child's optional-level floor routine music being used by half the other girls at the meets? You can create your own! With a few simple computer tools, practice using the software, and the time to edit it to perfection, you can create an original musical selection that no other gymnast in the world is using!
1. Identify the music you want. The best will feature changes in tempo and intensity. It cannot have any lyrics. Even one word, such as "go!" will result in an automatic deduction off every routine. Be aware of the gymnast's style and preferences, too. They will perform better to something that they enjoy and are comfortable with.
2. Get a music editing computer program and learn how to use it. There are some really good freeware software programs available to download free for individual, non-profit use.
3. Convert your music to the appropriate file type for editing with whatever program you choose. There are many freeware programs available that will do this for you.
4. Identify the specific portions of the music that you wish to use for the routine and note where they are within the music. You can do this simply on paper first, noting the time within the piece, and "naming" the different sections so that you can refer to them later and not get them mixed up. For example: :00-:23 Intro, :52-1:02 slower transition, 1:02-1:15 drums, etc.
5. Use these parts to make a "rough draft" or "storyboard" of what you want it to be when you're finished. Be sure to include different sections that would be appropriate for tumbling passes or more "dancey" sections of a routine.
6. Edit away! Make sure that your transitions are smooth and that the musical timing is not altered. Listen to it frequently as you go along and you will catch the places where it just doesn't sound right. As with all types of editing - audio, video, text - it is also very effective to "walk away" for a while. Let your work sit overnight and listen to it with fresh ears the next day. You will notice things then that you don't catch after you've been working with it for a while.
This is a slow process. Don't expect to sit down with no experience and edit-up a piece of music to professional quality. You will need to take the time to practice, learn, and then finally edit your chosen music to perfection.
Floor exercise music can not be longer than 1 minute 30 seconds, and it is safer to keep it a couple of seconds short of that to make sure no one questions the length. A lower level optionals routine does not even have to be that long, and you could cut it as short as 1:10.
Edit between five and ten different pieces of different styles to present to your coach and/or athlete and let them choose between them.
Discuss with your coach and find out what they would prefer in music: length, style, tempo, etc.
Be prepared to accept that a coach/gymnast may not like any of what you have to offer - even after you spent 100 hours editing it all together! This isn't necessarily a criticism of your editing skills, but part of the process of choosing the perfect floor exercise music for a specific gymnast.