Is there a library for sounds of UNKNOWN origin to help identify the sources of these sounds?
If you think your sounds are avian, then Xeno-canto also includes a 'Mystery recording' section, and I have had success in crowd-sourcing species IDs for recordings in the past.
If you have absolutely no idea what has made the sound, then that is definitely more difficult! I agree with @CamDesjonqu's comment, it would be great to have a resource for this.
For birds, a potential option is to try using BirdNET, from the Cornell Lab of Ornithology (ML model). Their goal is to be the "Shazam" for birds.
You would just have to ask them what the coverage for that geographical area is to see if it would be worth running it through. They have support for something like 3000 species now (with nearly 100 million detections), but there are blind spot areas (like Madagascar, as I have come across with my own work). It pulls from the Macaulay library a lot too (same maker, Cornell). The platform can be run on the site, in the app, or locally on your computer (Github page for this: https://github.com/kahst/BirdNET-Analyzer).
I agree this would be really cool & useful though! A kind of Wildlife Insights but for passive acoustics rather than camera trapping.
Might also be worth posting this in the WILDLABS acoustic monitoring group too as a complementary effort.