By using the Notification Sounds website you agree to the terms as listed below. If you do not agree to these terms, please do not use this site.
Unless stated otherwise, all the downloadable sounds are provided under a Creative Commons Attribution license.
That means the sounds are absolutely free to use and you can even freely use them in your projects (even in commercial projects) as long as you provide attribution by linking to the Notification Sounds website.
For instance in your project's “About” or “Credits” section (or on the accompanying website, if any, or on the GitHub's readme file if it's an open source project) you should link to our website with a text like “Sounds from Notification Sounds”.
If you have any questions or need further information please feel free to contact us.