The website freesound.org is a great place to start searching for free audio files. Extensions for products like Adobe Premiere Pro or After Effects are also available (see Freesound Audio Search Extension).
Yes. The Freesound Audio Search Extension is free. You do not need to pay for the extension or for downloading audio files.
Every audio file can have it’s own licence. Most files are licenced either under the public domain or under one of the creative commons licences.
The Freesound Audio Search Extension only allows searching for audio files. However you can download the Free Stock Image Search extension if you need free stock images as well.
There are several Creative Commons licenses. Some licences like the Creative Commons Zero or the Creative Commons Attribution allow you to use the work in commercial projects. To learn more about the licences visit the following site: About The Licenses – Creative Commons.