There are a couple of mistakes in the User Guide entry on adding audio in Notion (Chapter 15). In Score Setup, the guide tells you to hover over Special Staves and then select Basic Staff. There is no Special Staves that I can see.

Then to add an audio file it says to select Tools>Attach Audio File, but that's wrong. It's Score>Attach audio files.

FWIW, the audio capability, that is the ability to use audio clips in a score, is nothing to write home about.