I agree that this should be documented better but looking at their terms of service, the recordings submitted look to be under CC0 [1].

The relevant blurb:

    Your Contributions and Release of Rights
    
    By submitting your recordings, you waive all copyrights and
    related rights that you may have in them, and you agree to
    release the recordings to the public under CC-0. This means
    that you agree to waive all rights to the recordings
    worldwide under copyright and database law, including moral
    and publicity rights and all related and neighboring rights.

[1] https://voice.mozilla.org/terms