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Yes, commercial use is no problem. You can create end products that you can sell to clients and they can then sell that end product on.
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There are a couple of exceptions for when you’re using the sound effect in a series, but in general if you buy a multi use license then you can use it as many times as you like in as many different projects as you like. Otherwise, you need to buy a single use license every time you use it. Check the license FAQ though, for more details on using in a series.
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It is preferred that you credit AudioJungle and the specific author, where possible, but it’s not absolutely necessary.
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You don’t own the copyright to the audio, just a license to make use if it… but you do own the copyright of the production as a whole… as you have permission to use that audio in your production. So there should be nothing wrong with adding that.
One thing to note… this all relates to sound effects only, the rules are slightly different for other audio categories. There are varying licenses depending on the usage of the end product, so it’s slightly more complex than sound effects.