composing an original piece of work - if you are using someone else's composition it may be subject to Copyright and/or Royalty fees
using the Instruments to either write the notes (MIDI Editor) or playing those instruments using the computer keyboard or a MIDI keyboard
Then there are no royalty fees, or commercial licences needed to use.
However, if you use Soundpool loops in your composition, and are publishing for commercial gain, or on a site that supports adverts and makes a financial gain, eg Youtube, then you need Commercial licences for every Soundpool that contains a loop you use.