Two-Track Audio Mix plays two sounds at the same time and renders them into a single file — for example laying a spoken A track over a background-music B track. Track A is the audio currently selected on the left of the workspace and track B is dropped into the panel separately; either level can be set from 0% to 200%, with A at 100% and B at 60% by default.

The final length can follow “the longer”, “the shorter” or “track A”, and the output is MP3, M4A or WAV. The implementation converts both tracks to 44.1 kHz stereo, then overlays them with amix with automatic normalisation switched off, so setting the levels too high raises a warning about clipping.

1. 01Prepare track Aselect one audio file on the left as the main track, for example the voice.
2. 02Add track Bdrop a second audio file, such as background music, into the “Track B” area; only one file is accepted at a time.
3. 03Set levels and lengthset tracks A and B from 0% to 200% each, then choose the longer, the shorter or track A to decide the output length.
4. 04Choose a format and mixpick MP3, M4A or WAV, confirm the command, click “Start mixing” and download the result.

Track count

exactly two tracks, A and B; there is no third track, no start offset and no timeline placement.

Level range

0%–200% per track; amix runs with automatic attenuation off, so a combined gain that is too high can clip, and the interface warns when the total exceeds 250%.

Sample format

both tracks are converted to 44.1 kHz stereo before mixing, so this is not a lossless wrapper that keeps each source's own format.

Output formats

MP3, M4A and WAV only, and the result is audio — no video is produced.

What it does not do

to replace or add a soundtrack on a video use “Video Soundtrack Mux”; to join clips end to end use “Audio Cut and Merge”.

Both audio tracks are mixed only locally in the browser by ffmpeg.wasm and are never uploaded; the first use of the workbench downloads about 31–32 MB of engine from the site's asset domain or jsDelivr.