To play audio files on the KN2600 (or KN7000), they must have been copied to the card using the Panasonic SZ-CB7 kit which consists of a Panasonic Reader/Writer and SD-Jukebox software. The Jukebox software is required to format the audio files in a form that is recognised by the keyboard and it will only work with the Panasonic writer.
I doubt that any of the dealers still have the kit although you might try Walt Tenay. They may occasionally turn up on Ebay or the for sale section of the general synthzone forum.

Cheers
Len C.