My sound font converter assumes you are converting a Plecter font, so all of the file names have to match Plecter default names. The name of the file that came with the board may not match what Plecter uses, so you may have to rename it.
Also, make sure all of your sounds are the same format as Plecter uses. Off the top of my head, I think that is 22050 KHz, 16-bit, mono.