Kdenlive/Manual/Project Menu/Online Resources
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Online Resources
Available from the Project menu and the Add Clip dropdown, this feature allows you to search online audio, video and graphics libraries for resources to download.
It looks like the freesound audio library part of this no longer works - Oct 2015. Probably related to the fact that freesound now requires user registration. You can still download great audio clips from https://www.freesound.org/ - just not through the Kdenlive interface.