It replaces the original sound, it does not sit under it
Choosing a track means the original audio is never written into the new file at all, which is why there are no levels to balance and nothing to duck — there is only ever one audio source in the result. The Add Music to Video is deliberately narrower than a real mix for that reason: balancing two tracks properly is a different job, done in different software.
What happens when the lengths do not match
A track longer than the video is trimmed where the video ends, with a short fade-out so it stops rather than cutting off mid-note. A track shorter than the video plays out in full and the remainder of the video is left silent, rather than looping the track back to its start — a loop is almost always more noticeable than a quiet ending, so it is not offered. A track that happens to finish naturally within the video’s length keeps its own ending either way.
The real risk here is copyright, not audio quality
A commercial track added to a video that gets posted publicly is a copyright claim waiting to happen, and the usual outcome is the post being muted, blocked or removed after the fact rather than before. The music libraries built into social platforms are licensed for exactly this use; a track from a personal library or a purchased album generally is not, however incidental the use feels.
The start point is worth setting deliberately
Left at its default, playback begins at the very start of the track, and most tracks take fifteen to twenty seconds to reach the part actually worth hearing. A short clip that opens on a track’s intro sounds like nothing is happening yet. Finding the section that should play and setting the start point there is usually a better result than leaving it at zero.
Turn it down under a voice-over
Somewhere around thirty to fifty percent volume sits under narration without competing with it. At full volume the music becomes the content rather than the backing, which suits some clips and works against most of them where someone is actually talking over it.
Why this does not work at all in Firefox
MP4 audio needs the AAC codec, and AAC encoding is unavailable in Firefox on every platform and in every browser on desktop Linux. Because writing an audio track is the entire purpose of this tool, rather than an optional extra on top of the video, there is nothing useful it can do where AAC is missing — unlike a tool that merely drops the sound, this one has nothing left to offer. Chrome, Edge and Safari all work normally.