[bebio]

I particularly like how in JCHI they go from Shoujo Robot to Kabuki to Himitsu in one go.

This thread reminds me of those "mash-ups" that were so popular a few years ago. Especially 2ManyDJs were very famous doing that.

If you want to reverse a song, get an audio editing program.
I use Adobe Audition, but maybe Audacity (which is free software) can do the trick.

You have several tracks in the audio editing program.
drag each audio file into its own separate track. then slide one of the audio "blocks" to the position where you want to make the transition. then you use the volume sliders that exist in each "block" and you lower the volume in one block, and raise it on the other to create the transition.

If you want to reverse an audio block, select all of the block, and search for an option called "Reverse" (I don't know if Audacity has it, but it's likely), and apply it to the audio block. There, you now have a backwards song.

In the end, just export the result to a .wav file and convert it to mp3.
If this is too complex, maybe I'll make a tutorial in the future. But it's not rocket science.