How can I download and edit/shorten the free to use music clips?
The free-to-use clips all come from the YouTube Music library. You can access them and search for them by name at the YouTube Music Library playlist:
Also, if your goal is just to shorten the imported audio file, then you can do that within the Soundslides application.
In your project (using the Timeline editing mode), click on the Project tab, then check the box for “Adjust audio duration”. You can then shorten the audio duration by entering in your desired length.