Check out the music on this site, and once you have found it, click the License link, then check to see if your video qualifies as "non-commercial". If it does, you can probably use it royalty-free (subject to the conditions stated on the page), but I would recommend contacting the site to double-check.

https://magnatune.com/

Note: There is a separate "Video" license type listed, but the non-commercial aspect of most Flight Simulator videos may make the "Non Commercial" license more appropriate. If in doubt, contact the site. The advice given here is speculative. You alone are responsible for ensuring you have the correct license.