Is it legal to use the music and sound effects that come with iMovie?

Hi,

You may want to study the applicable licence agreements (for example, https://www.apple.com/legal/sla/docs/iMovie.pdf).

The participants here are users, just like you, so for any questions, you would have to contact Apple and its legal department (Legal - Contact Us - Apple).

Yes, they can be used for any private or commercial project.