Most naturally occurring sounds are not copyrightable; only sounds with human creative authorship can hold copyright

Copyright protects original creative works of authorship. Most natural sounds — rain, birds, traffic, wind — have no human creator and therefore no copyright owner; they can be recorded and published freely. Human-made sounds (a musical performance, a spoken dialogue, a sound effect from a film) may be copyrightable. Even within human-made sounds there are edge cases: recording a crowd or street vendor in a public setting is generally permissible as long as no single identifiable person is recorded without consent. When in doubt, recording street noise, weather, animals, and ambient environments is the safest creative-commons-compatible approach. Freesound explicitly states that ‘most of the time only human-made sounds can be copyrighted.‘

Examples

Acceptable Freesound upload: a field recording of ocean waves. Not acceptable: a recording of a DJ set or a commercial song on the radio. Edge case requiring judgment: a recording of a street busker (their performance has copyright; ask permission).

Assessment

A sound designer records ambient traffic noise, then separately records herself humming a melody. Which of the two recordings does she hold copyright in? Can the traffic recording be uploaded to Freesound under CC0?