Hi Benni,

I haven't used the MS-40, but I've seen a few sites selling them that indicate the sounds can be edited.

"128 USER VOICES

The 256 Internal Voices of the MS 40 can be edited by means of the Attack, Decay, Release, Cut Off, Resonance, Tune, Octave and Modulation parameters. The User Voices can be memorized in 128 internal memory locations or saved on Disk, and when in the PLAY mode perform in exactly the same way as the Standard Preset Voices, providing alternative sounds for the making of new Programs or Patterns/ Styles."

https://reverb.com/ca/item/5148594-solton-ketron-ms-40-midi-synthesizer-sound-module-sound-floppy-256-voices-128-user-12-drum

I haven't been able to find a manual online, but I'm thinking you'd start by pressing the Edit Voice button near the top of the device.

Thanks
Kent
PG Music