Increase Video Volume.

Apply gain to the audio track and copy the picture across, which is the whole job and takes seconds here. The number to watch is the peak: every decibel of gain moves the loudest sample up by a decibel too, so a clip already peaking near full scale has almost no room and pushing it anyway produces clipping rather than loudness. This shows the peak before and after, holds the gain at whatever keeps the loudest sample under minus 1 dBFS, and says when it did. If the clip is still too quiet at the ceiling, the recording itself has a wide dynamic range and what it needs is compression rather than gain, or a re-record closer to the source.

LUFS is Loudness Units relative to Full Scale: a measurement of how loud something sounds rather than how big its numbers are. A quiet solo voice and a dense music bed can peak at exactly the same sample value and be nowhere near as loud as each other, which is why peak meters are useless for this and why the broadcast world agreed on a perceptual standard instead. The meter weights the signal roughly the way an ear does, averages it over the whole programme, and gates out the silences. Minus 14 matters because that is roughly where YouTube, Spotify, TikTok, and Instagram normalize everything they play: upload something louder and it is turned down for you, upload something quieter and on most of them it is left quiet. Landing on minus 14 yourself means the platform does nothing and your clip sits at the same level as everything around it.

Because the peak got there first. Loudness and peak are two different measurements, and applying gain moves both of them by the same number of decibels. A clip that averages minus 25 LUFS but has one door slam peaking at minus 2 dBFS cannot be turned up eleven decibels without that slam landing nine decibels past full scale, which is distortion the file cannot store. So the gain is held at whatever keeps the loudest sample under minus 1 dBFS, and the tool tells you the gain it applied and the loudness you will actually land at instead of pretending. If you need to close the rest of the gap, the honest fix is compression or limiting on the original mix rather than more gain here.

No. The video packets are copied into the new file without ever being decoded, so the picture in the output is bit for bit the picture in the source: same resolution, same frame rate, same codec, same generation. Only the audio track is decoded, multiplied, and encoded again. That is why the export is fast on a large file, and it is why you can run a clip through here after grading or editing without giving up a generation of quality. The one exception is a file the browser cannot parse at all, such as an AVI, which is converted once with the fallback decoder first, and the tool says so before it starts.

Yes, and it stays what it was. An MP3 comes back an MP3, a WAV comes back a WAV at 16-bit PCM, and an M4A comes back in an MP4 container. The channel count and sample rate are kept, the same BS.1770 meter reads the file, and the same targets and peak ceiling apply, so a podcast episode measured at minus 21 LUFS can be brought to minus 16 for Apple Podcasts in one pass. A lossy file is re-encoded to carry the new levels, because there is no way to change the samples without writing them again, so the export is one generation on from the source. A WAV is not, since PCM is written rather than encoded.