It could be on the B-side of a cassette called Toolbox. It came with my C64 I think, and told you how to set up the hardware etc. At the end, the guy says something like "Why don`t you check out the Amiga range from Commodore, amazing graphics, wonderful sound etc etc... You can hear some music made on the Amiga on the other side of this tape"
It was a good tune IIRC. Think I still have the tape somewhere...
Hmmm....
