They do have the disclaimer below –
LibriVox recordings are Public Domain in the USA. If you are not in the USA, please verify the copyright status of these works in your own country before downloading, otherwise you may be violating copyright laws.
And interestingly audio is provide not in only in mp3 format but also in the Open Source ogg vorbis format.
With Google providing so many public domain books with enough volunteer readers there could well be an excellent collection at LibriVox.