Its calls mono /path/to/NzbDrone.exe.
What may work for you is to explicitly add mono to the path via the script that starts Sonarr. This concept is used in a lot of applications to set environment variables that are required for the application to function (Hubot comes to mind, as you set passwords and such via that mechanism).
On Ubuntu you can use: export PATH=/path/to/dir:$PATH I’d expect it to be something similar on QNAP.