CC0 waives all rights; CC-BY requires credit; CC-BY-NC additionally bars commercial use
The three current Freesound license tiers have distinct requirements. CC0 (zero) is the most permissive: the creator waives all rights so the sound can be used in any context including commercial resale, but the downloader may not falsely claim authorship. CC-BY (attribution) requires crediting the original creator in any published work that uses the sound. CC-BY-NC (attribution noncommercial) adds the constraint that no revenue may be earned from the work containing the sound. In all cases the original licensor retains the right to grant additional permissions outside the stated license. Understanding these three tiers is the minimum prerequisite for legally sourcing samples from Freesound.
Examples
Using a CC0 sound in a paid game: legal. Using a CC-BY sound in a free podcast without crediting the author: not legal. Using a CC-BY-NC sound in a monetized YouTube video: not legal without permission.
Assessment
A producer wants to release a commercial EP that samples a Freesound loop. Which license tier(s) allow this without contacting the uploader? Which tier(s) require attribution in the liner notes even for a non-commercial release?