Replace the Audio in a Video, Every Setting: Mix, Loop, Fades

Every setting of the free Replace Audio tool on a 15-second clip with a voice note over it and a 20-second music loop. Soundtrack replaces the sound (fades 0.5 / 1.5 s); Background music keeps the voice and mixes the music under it at 35%; Voiceover plays a recording once. The picture is copied byte for byte every time. When the audio is shorter: loop it, play it once, or cut the video to it (6 s). Trim, offset and fades; MP4, MOV or MKV (WebM would re-encode the picture, so it is off). You hear the before and after. Nothing is uploaded.

Replace the Audio in a Video, every setting, on a 15-second 1080p clip whose sound is a voice note, with a 20-second music loop to add. The Soundtrack preset replaces the sound, fading in over half a second and out over one and a half; Background music keeps the original voice and mixes the music under it at 35%; Voiceover plays a recording once at full volume. Every export copies the picture byte for byte — only the sound changes — and you hear the before and after. When the audio is shorter than the video, it can loop to fill, play once and leave the rest silent, or cut the video to the audio (6 seconds here). Trim picks which part of the audio is used, offset shifts where it starts, and fades soften both ends. MP4, MOV and MKV keep the picture as it is; WebM would need it re-encoded, so for this clip it is switched off with the reason shown. Free: one clip up to 200 MB and 10 minutes, audio at 44.1 kHz. Pro: 20 clips with the same soundtrack, at 48 kHz. Nothing is uploaded.