Convert an old .aup project to MP3
An .aup file from Audacity 1.x or 2.x holds no audio by itself — the sound is in the _data folder saved next to it. Drop the folder that holds both, or a .zip of it, and download the mix as MP3 or WAV, with nothing uploaded.
An Audacity 1.x or 2.x project (.aup)? — the .aup and its _data folder together, or drop that folder or a .zip of it above.
How do I turn an old Audacity .aup file into an MP3 without Audacity?
Keep the .aup file and the folder next to it that ends in _data together — the .aup is only the list of the project's audio blocks, and the _data folder holds the audio. Drop the folder that contains both (or a .zip of it) on this page: the browser reads the block files, places each clip where the .aup says, and encodes an MP3 locally, with nothing uploaded. Audacity 4 cannot open .aup projects; Audacity 3.x can, by converting them to .aup3.
Drop the folder that holds the .aup and its _data folder — or a .zip of both — or choose the folder with the button under the drop zone.
See the tracks and clips and play the mix; any missing block files are listed.
Pick a bitrate and download the MP3 (or choose WAV).
Pro adds per-track exports and exports of projects larger than 256 MB.
Convert an old .aup project to MP3
These are the projects on old laptops and backup drives from before Audacity 3.0, and they are the hardest kind to rescue: Audacity 4 cannot open them, and a converter that sees only the .aup sees a small XML file. Everything needed is usually still there, though, as long as the _data folder survived beside it. This page reads the two together — the document for where each clip goes, the folder for the audio — and mixes them in your browser.
If you only have the .aup, it still opens and tells you exactly which folder the audio was in and how many block files it is looking for.
On a Mac? Audacity's own recovery utility only gained a macOS build in January 2026, and it is still a terminal program that asks for about four times the project's size in free space. Everything on this page runs in the browser instead, on any operating system.
What is actually inside an Audacity 2 project
The .aup file is plain XML. Its root names the project folder in projname (for a project called song, song_data), and each wavetrack holds clips (waveclip, with an offset in seconds), each clip a sequence, and each sequence a list of waveblock elements. A waveblock does not contain audio; it names a block file — simpleblockfile filename="e0000c97.au" len="1544" — that Audacity stored inside the _data folder, spread over e00/d00-style subfolders.
Each block file is a Sun .au file written in the byte order of the machine that saved it, which is why a project saved on a PC starts its blocks with the letters "dns." — the standard ".snd" magic written backwards. The audio does not start right after the 24-byte header: Audacity puts the summary data it uses to draw waveforms there first, and the header's data offset skips past it (3,128 bytes into every block of the Audacity 2.1.0 project this page is tested with). The data-size field is left at 0xffffffff, so the length comes from the file size.
Two other block types matter when a project is rescued. A silentblockfile is just a length of silence. A pcmaliasblockfile is a block whose audio was never copied: it records the full path of the original imported file and where in it the block starts. Those are the gaps that appear when an old project is moved to another computer without the files it was built from.
- Used by
- Audacity 1.x and 2.x, until 3.0 (2021) introduced .aup3
- Project file
- Plain XML (.aup) listing tracks, clips and block files
- Audio
- In the projname_data folder, as many small .au block files
- Block file header
- Sun .au in the saving machine's byte order — "dns." on a PC
- Sample formats
- 16-bit, 24-bit (three bytes each) or 32-bit float
- Opened by Audacity 4?
- No — Audacity 3.x converts it to .aup3 first
- Opened here
- The .aup with its _data folder, or a .zip of both
New to the format? What an .aup4 file is, table by table — and how it differs from .aup3.
Sources
Where the facts on this page were checked. Audacity's code is cited at the tagged release it was read from.
- SimpleBlockFile.cpp, Audacity 2.4.2 — A block file is a Sun .au file whose magic 0x2e736e64 (".snd") is written in the machine's own byte order, so a little-endian PC writes it as "dns."; dataOffset is the header plus Audacity's summary bytes; dataSize is written as 0xffffffff; the encoding is 16-bit, 24-bit or 32-bit float, and 24-bit samples are written three bytes each.
- PCMAliasBlockFile.cpp and SilentBlockFile.cpp, Audacity 2.4.2 — A <pcmaliasblockfile> records aliasfile (the full path of the original imported file), aliasstart, aliaslen and aliaschannel instead of holding the audio; a <silentblockfile> records only len (SilentBlockFile.cpp, same folder).
- ExportMP3.cpp, Audacity 3.7.9 — Audacity's MP3 export mixes to floatSample and calls lame_encode_buffer_interleaved_ieee_float (or lame_encode_buffer_ieee_float for mono), so LAME receives 32-bit float samples rather than 16-bit integers.
Converting this file
Q. I only have the .aup file. Can it become an MP3?
Not by itself: an .aup is a text document listing which block files make up each clip, and the audio is only in the _data folder whose name is in the file's projname attribute. Drop the .aup here anyway — it opens, shows the tracks and clips, and says which folder and how many block files are missing, so you know what to look for on the old drive or in the original upload.
Q. Some parts are silent even though the _data folder is there. Why?
Audacity 2.x could import an uncompressed file without copying it into the project, recording a pcmaliasblockfile that points at the original file's full path instead. That audio was never in the _data folder. This page counts those blocks and names the original files, so you can find them; the parts they cover are silent in the mix.
Q. Can Audacity 4 open my .aup project?
No — Audacity 4 opens .aup3 and .aup4 projects but not the older .aup format. Audacity 3.x still imports an .aup and converts it to .aup3, which Audacity 4 can then open. To get the audio out without installing anything, use this page.
Q. Is the audio changed on the way out?
The block files are read as stored — 16-bit, 24-bit or 32-bit float — and mixed with each clip's offset, the track's volume and pan, and its envelope. A WAV export in 32-bit float hands that over unchanged; MP3 and 16-bit WAV are rounded to 16-bit without dither. Realtime effects are not part of this format and nothing needs rendering for them.
Free to open, play and export the stereo mix for projects up to 256 MB; larger projects still open and play free, and downgrading to .aup3 is free up to the same size. Recovery, per-track stems, and exports or downgrades above that size are a one-time $19 license — no subscription, no recurring fees.
Every job an Audacity project needs
Same local-first engine, one page per task.