Merge takes into one file
Pick the files, put them in the right order, and get one MP3 back — or one M4B audiobook with a chapter marker at every part. This audio joiner reconciles parts that disagree about sample rate or channel count, tells you where each seam lands and plays the join back, all inside this tab with nothing uploaded.
Ready.
What it does with your files
Real books arrive uneven: a mono chapter beside a stereo one, at two different sample rates. Every part is brought to the first file’s channel count and sample rate — 44.1 kHz if the encoder cannot write that rate — before anything is concatenated. Joining them as they arrived would produce a file that plays at the wrong speed — a failure that sounds like a broken encoder and is not.
Chapter two before chapter one is a broken book. The list shows the order the parts will be written in, and every part can be moved or removed before you commit — without reloading and picking them all again.
The result lists the timestamp each new part starts at, so you can jump straight to a join and check it rather than listening to the whole file hoping.
Choose M4B and the parts become one audiobook file with a chapter at each part, named after its file, so a player can skip from chapter to chapter. The chapters are written twice — as the QuickTime chapter track Apple’s players read and as the Nero chapter list many other players read — and read back from the finished file before it is offered. The audio is AAC, encoded by your browser’s own encoder at 96 kbps mono or 128 kbps stereo. Not every browser has one: if yours does not, the page says so and the MP3 join still works. No title, author or cover is written.
Every file is decoded, joined and re-encoded inside this browser tab. The tools on this site never upload your audio, which is why a manuscript under embargo can be worked on here at all.
Publishing it?
Audiobook platforms judge each file on its own numbers, and joining does not change them. These tools measure what you end up with, and none of them uploads anything either.
The chapter checker measures a finished file against ACX, Spotify, Google Play and Apple, and the batch checker does a whole book at once, flagging chapters that disagree with each other.
The noise remover and the trimmer are the two jobs that usually come before joining takes into one chapter.
Straight answers
Pick them all at once, check the order in the list, and press join. Every part is decoded in this browser tab, brought to the first file's channel count and sample rate (44.1 kHz if the encoder cannot write that rate), written end to end, and encoded as one 192 kbps MP3 — or as AAC in an M4B, if you choose one. Nothing is uploaded at any point.
Yes, and that is the part worth knowing about. A mono take beside a stereo one, or files recorded at 48 kHz and 44.1 kHz, cannot simply be written end to end — the result plays at the wrong speed. Each part is converted to match the first file before anything is joined, and the page says so when it had to.
Yes. The list shows the order the parts will be written in and each one can be moved up or removed before you commit. A joiner that cannot be reordered has to be reloaded to fix a mistake, which is the same as starting again.
The limit is your own device's memory, because the work happens there. Compressed sources such as M4A are decoded whole and capped at about forty mono-minutes each; WAV, AIFF and MP3 stream in chunks and have no such cap.
Yes. Pick the chapter files in order, choose M4B, and join: each file becomes a chapter named after it, in an M4B whose chapters are written both as the QuickTime chapter track Apple's players read and as the Nero chapter list other players read. The audio is re-encoded as AAC by your browser's own encoder, so the page first asks the browser whether it can; where it cannot, it says so and offers the MP3 join instead.
An M4B holds AAC audio, and a web page can only encode AAC if the browser ships an encoder for it. That depends on how the browser was built, and some builds, Firefox among them, have none a page can use. The page asks before it starts rather than failing part-way through, and nothing about the MP3 join depends on it.