Why? Music sets the mood and makes it less awkward to be on stage or talk. The silence doesn't seem to last as long if you have music in the background.
But either if you are posing, dancing, miming/singing or holding a speach, you might want to edit that song. Maybe it is too long or you only want the last part, or you want the middle part to fade in and out.
How? For sound editing I use Audacity. It is a free program and it is fairly simple to use. (tutorial here)
The sound samples I use I either take from games, from youtube (any youtube to mp3 converter will help you there) or record them my self. And of course I use original soundtrack. Please bear in mind the copyrights when mixing music and where you are going to use it.
But you have more possibilities than just using a plain song!
In Norway you are only allowed to be one on stage, and monologues can become a bit dull if you don't do it right. So prerecording a dialog either that you can communicate with or lip sync to might be a way to go. Or maybe you wear a hemlet and can't speak live.
And you can add more effect sounds, like explosions or magic or the sound of footsteps.. This makes the performance come more to life.
The Results! I have two examples for you from my own stage performances: I am in no way an expert on this, just showing you what I know, but I have gotten great feedback on from the judges and audience on these two.
First example is my Vi performance from League of Legends. Here my performance concept is a commentator reviewing a character from LoL (in this case Vi) while showing off her moves and quotes. So I had a friend read in the presentation monologue and I added the music in another layer in Audacity. That way I could change the "music volume" without changing his voice. For the quotes I added "silent" slots so I could speak them live.
And here is the performance.
For my second example I will use my latest performance with the Adjutant. This character only comments on actions in-game, so that was my angle for the performance concept. I wanted the performance to be "a player logging in to the game and starting a tutorial" with me as an adjutant commenting. Then is the "problem" with the adjutant having a robot voice. Hence I had to prerecord my speech and run it through a robot voice change filter (on my mobile phone actually, there are tons of apps for this out there)
The final soundtrack file is several layers of sound samples.
1. The Starcraft login music
2. Me hitting my keyboard to simulate a "Player" playing the game
3. My prerecorded robot monologue
4. In-game sound effects.
So many layers in audacity.
For this I had to lip-sync and then it is important to really know the soundtrack to get the timing right. And it is a tip to overdo the lip movement (or it will disappear on stage). Here is my performance. Not perfect with syncing my lip movements, but again, this was the first time I tried doing something like this ;)
Hope this helped a bit, just some random thought about how I prepare for a stage performance.