AUP3 stands for Audacity 3 Project File created by Audacity 3.0.0 and later. In a nutshell, AUP3 is a new project file format that replaces AUP format used by Audacity old versions. By the same token, AUP3 can be opened and used by Audacity 3.0.0 and later only, no other software and application can read and play it.
Unlike AUP file, which is a XML file that only contains references to the real audio files, edits, other data and coordinates with "_data" folder, now those needed files and project data are all housed into one large database .aup3 file to eliminate the "file missing" or "corrupt project" issue. In other words, it's much easier to save and share your project within a single .aup3 file.
Converting AUP3 to MP3 is actually equivalent to rendering AUP3 project to real MP3 audio file. Before we get started, you're required to update Audacity to 3.0.0 or later version from official website: https://www.audacityteam.org/, as AUP3 file CANNOT be opened in versions of Audacity prior to 3.0.0.
Step 1. Launch Audacity, click "File" menu > "Open" (or press the hotkeys "Ctrl + O" on your keyboard) to locate and load your AUP3 file. Alternatively, if it has already been associated with Audacity, just double-click the AUP3 file to directly open it in Audacity.
Step 2. Audacity will restore your unfinished work in the in the last editing. Now you are able to go ahead with your music production work, such as apply audio fade in/out, isolate vocals, normalize audio, etc., or proceed to convert AUP3 to MP3.
Step 3. Since LAME MP3 Encoder has been included in Audacity, there's no need to configure it separately. Just click "File" > "Export as MP3". On the "Export Audio" window, you're allowed to choose a destination path, rename the file, change MP3 bitrate and select channel mode. Then press "Save" button. You can also choose "Export as WAV" or "Export as OGG" as you wish.
Step 4. Here you can edit MP3 metadata information, and lastly, hit "OK" to convert and export AUP3 to MP3 audio file.
Optional. If you're more inclined to other audio formats like FLAC, MP2, WMA, M4A, AC3, etc., click "File" > "Export" > "Export Audio...". However, it requires you to configure the optional FFmpeg library to implement it. We have discussed the specific steps to install FFmpeg Library for Audacity AAC import/export, so it's unnecessary to go too deeply into this again.
There is no way to do that, as the official website states once a project has been saved in the unitary AUP3 project file it can no longer be opened in earlier versions of Audacity. If you insist on AUP file, you have to convert and export AUP3 to WAV, MP3 or other audio file, then open it in Audacity 2.4.2 or early versions to save your unfinished editing in .aup format. But conversely, you're able to open and load the old .aup file in Audacity 3.0.0 and later versions, and save the project in AUP3 format.
Many people seem to complain Audacity failed to open AUP3 file saved from previously opened AUP file or network drive, someone even ran into Audacity crash or freeze while opening AUP3 file. Since AUP3 is a new project file format, there will inevitably be a lot of problems. I've rounded up some possible solutions below for your reference.
1. Project is in a read-only directory. Move the AUP3 file to a different directory and try again.
2. Disable firewall/antivirus like Windows Defender. Or add AUP3 file to white list.
3. Audacity blocks you from using FAT/FAT32 disks for opening/saving project files as they are limited to a maximum 4GB per single file and this can easily be exceeded when editing.
4. DO NOT open and save active project on an external USB stick/disk, networked storage or cloud storage as it is unlikely to be fast enough for satisfactory recording and editing.
5. Ensure the project is fully saved before exiting Audacity, otherwise, the AUP3 file would be corrupted.
6. Be patient for importing AUP3 file to Audacity because AUP3 files contain all data, they can be very large (several gigabytes) and may load more slowly than AUP files.
That's all about the brief info of AUP3 file and how to convert AUP3 to MP3 and other formats in Audacity. I have to say that AUP3 file does make it easier to move, rename, copy and share Audacity projects without worrying about original audio missing or incorrect file path issue. Meanwhile, as you know, the emergence of new things is always accompanied by minor flaws and instability, but they'll be fixed in the future releases of Audacity. Thanks for reading!