Sound is a distribution signal, not just decoration
On several short-form platforms, using a sound that is currently trending can help a clip get surfaced, because the platform groups content by audio. That makes the sound library a distribution tool, not only a creative one. The catch is that this effect is strongest with audio the platform itself is promoting.
The in-app library is the safe default
Each platform's built-in audio library is licensed for use on that platform. Picking a sound from inside the app is the lowest-risk option because the rights question is already handled for that context. The limitation is that those sounds are tied to the platform and may not be usable if you cross-post the same clip elsewhere.
External music is where rights get sharp
Music you add from outside the app — a track you like, something from a download site — is only safe if it is explicitly licensed for the use you are making. "Free to download" is not the same as "cleared for a commercial clip on a public account". Getting this wrong can mute a clip, suppress it, or expose you to a claim.
For anything that is a brand or paid program, use royalty-free or properly licensed audio and keep proof of the license.
Sound effects and pacing
Beyond music, small sound effects — a whoosh on a cut, a subtle emphasis under a punchline — sharpen pacing when used sparingly. Overused, they read as amateur. The goal is a rhythm the viewer feels rather than notices.