$12256 / $11500
I've run across some free sound effects scattered across the web during my audio searches, but I've always hesitated to utilize them in the creation of my own sound effects. My question is: can free sound effects be legally combined with other sounds to create new sound effects? If so, does that in any way depend on the license under which the free sounds are distributed under?
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It depends entirely on (1) the license, or (2) direct permission from the copyright holder. An open license is simply passive permission from the copyright holder.
If a site just says "Free Sound Effects" then you can't assume permission to remix it.
I remix many open licensed sounds for OSARE. Because I'm releasing my work under GPL and CC-BY-SA, I'm safe to use Public Domain, CC0, CC-BY, and LGPL licensed works. I can also use works that are released under *both* GPL and CC-BY-SA. I don't think I can use items under the CC Sampling Plus license (I don't think it's GPL compatible).
Also, don't forget the option of asking the creator for permission. When you find some absolutely perfect sample, it can't hurt. Instead of asking "can I remix this?", ask "can I remix and release this under these specific licenses?".
I wouldn't risk using anything that just says "free" without asking for clarification exactly what that entitles you to do. If it's clearly available under a license that specifically allows it, go ahead, but make sure you comply with the terms.
We'll be happy to help you with more specific questions. I recommend to always keep information on where you found files if you intend to publish or make derivates of them later.