Understand that this was unheard of back in the days of the Commodore 64. According to the demo you are about to see the authors of Cubase said this couldn’t be done, that you needed more computing power. Pex Tufvesson proved them wrong.
Here is the main Cubase64 Website
http://www.livet.se/mahoney/cubase64.php look for the cubase64 link in the middle of the page. He documents everything on how he did this with code on his website.
You can get the software from Pex Tufvesson’s website: http://www.livet.se/mahoney/index_download.php
He also has a MP3 player for the Commodore 64