I stumbled on this today and just figured I'd share it. I've read the SQLite As An Application File Format page and read some on the forums here, but it's neat to see a live example of someone switching over to it.

https://www.audacityteam.org/audacity-3-0-0-released/

.aup3 Project Format

Audacity 3.0.0 is a major update on our previous Audacity 2.4.2. We’ve changed the format in which we save Audacity projects! Previously we saved projects as a sometimes large number of small files, with an ‘.aup’ file to coordinate the lot. This way of doing things is sometimes called ‘pile of files’ storage.

The problem, which happened all too often, was that data files and .aup file parted ways. Users quite reasonably expected the .aup file to contain the entire project. Well, the new .aup3 file does contain the data as well. The technical detail is that we are using an open source database, SQLite3, to store everything in one .aup3 file. That all happens ‘behind the scenes’. SQLite3 is open source, and it is a delight to work with. Nevertheless, this was a huge change, and we decided it was too risky to include many other changes we wanted to make at the same time – so 3.0.0 is almost entirely about this big format change.

Working with .aup3 projects editing audio should on most machines be a little faster than before, because there are fewer files being worked on. Finishing and closing a project at the end of working can be quite a lot slower, since there is more to do when a project is closed. We think the trade offs are worth it.

Importantly note that you can open your older .aup projects in Audacity 3.0.0 where they will be converted to the new .aup3 format.