Good guess, you must be running an old firmware version on your Pisound.
See here for info on upgrading the firmware: Pisound + Raspberry Pi 5: Hanging MIDI Notes - #6 by Giedrius
The scripts are not compatible with Pi 5 or bookworm OS, to make the upgrade, please use Pi 2, Pi 3 or Pi 4 and use the older Raspberry Pi OS, based on bullseye or buster Debian versions.