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Internet Archive: Open Source Audio
A huge collection of files, many of which are licensed for royalty-free use or are out of copyright
Many of the recordings uploaded to this site are available for royalty free use subject to Creative Commons licenses or due to being out of copyright, but the onus is still on you to check if you find something you may be interested in using:
Fascinating to browse even if you are not looking for anything specific!
Scotflight for FSX version 1.1 released