The Soundcloud Download extension does not collect any data.

Tracks are downloaded using Soundcloud's API, the extension never communicates with an external server. At runtime, the extension listens to XHR requests in the soundcloud.com domain and filters those made to the tracks, users, or playlists endpoints. This data is needed to find the download URL and metadata of tracks. The extension also parses out your client id, a unique identifier that is needed to authenticate requests to the Soundcloud API.

This extension also needs to listen to browser activity on navigation, and this is used to append the download button to every track page that is navigated to. Without this permission, you would need to reload the webpage whenever you navigate to a new track in order to re-activate the extension.

In order to delete all of the runtime data of the extension, it can simply be removed. The code for this extension is open source and can be viewed here:
https://github.com/Moebits/soundcloud-download

If you have additional questions or concerns, you can contact me on the github repository.