When I trialed Pre15, I didn't think that the Music Remix feature worked very well. Now that I have worked with it for a while, I see that I wasn't using it correctly.
The first time you remix a music file, by dragging the clip end to a new position, the remix has a definite beginning and ending to the song. But if you drag the clip end another time to a new position, the end of the song is now replaced with music from some other section. It's not remixed for the new duration. In the beginning, I just gave up at that point. The way to complete the remix and get the ending into the song is to click on the remix icon or double-click the clip to bring up the Remix dialog. The Remix slider then has to be moved to another position, and then you click Done. You can't just say Remix with current settings. If you want to move the Remix slider back to the original position, you bring up the Remix dialog again, move the slider, click Done again. Of course you could start from scratch and insert the whole song on the Music track again and resize once.
I didn't realize that you could mix your own music. I thought Adobe was talking about remixing the sound track music they provide, but I later found out that they are talking about remixing your own music audio files (.WAV, MP3, etc.). There are two remixing solutions: one for the sound track music, and another for remixing your own music.
Adobe also provides sample music (not sound track music) that can be used with the Music Remix, and that seems to work very well with the system.
That music is located in a hidden folder, C:\ProgramData\Adobe\Elements Organizer\15.0\Music, although it should show up in the Elements Organizer.
There are some MP3 files of my own that I tried, but after the remix, the music was sped up over 300% each time I remixed it. I had just read in Steve's guide and was reminded to convert audio files to WAV format for best performance and least headache. After I converted the MP3 to WAV, the music was remixed with about 100% speed.
To me the Music Remixer is the best new tool of Pre15. Some songs, though, are just not suited for remixing, and you'll find out what those are after you experiment with remixing songs for a while.