LibriVox 

Librivox audio is public domain (they use a Creative Commons Public Domain Dedication).  They use ebooks obtained from Project Gutenberg.  Many Project Gutenberg ebooks are also public domain (not all).  To make sure that they only release audio readings of public domain texts, Librivox relies on Project Gutenberg's legal work to assure Copyright status of their books.  

Project Gutenberg

The Acoustic Model creation process requires that we segment any user submitted audio (and its corresponding text transcriptions) into 5-10 second speech audio snippets.  But, in doing so, we would contravene the Gutenberg Project's Trademark licensing terms if we kept any references to Gutenberg in the eText that accompanies the speech audio.  For this reason, we need to remove all references to Gutenberg in any speech audio and text submission made to VoxForge.

To distribute Project Gutenberg e-texts with the "Project Gutenberg" trademark name, you must follow some licensing provisions that include a requirement that the text not be broken up in any way, and pay a licensing fee.  If  you don't use the Project Gutenberg Name, and delete any reference to it in the text, you can distribute the text in any way you see fit.

For example, for the Herman Melville book 'Typee', the Librivox audio is public domain (it uses the Creative Commons Dedication).  The text of the Gutenberg Typee ebook has the following "license".  It says the following in the intro:

ABOUT PROJECT GUTENBERG-TM ETEXTS
This PROJECT GUTENBERG-tm etext, like most PROJECT GUTENBERG-
tm etexts, is a "public domain" work distributed by Professor
Michael S. Hart through the Project Gutenberg Association at
Carnegie-Mellon University (the "Project"). Among other
things, this means that no one owns a United States copyright
on or for this work, so the Project (and you!) can copy and
distribute it in the United States without permission and
without paying copyright royalties.

Then it goes on to say: 

Special rules, set forth
below, apply if you wish to copy and distribute this etext
under the Project's "PROJECT GUTENBERG" trademark.

So basically it says that no one owns copyright on the written text of this book in the US (and likely most other jurisdictions), and you can copy and distribute as you please.  But, if you want to copy and distribute the book along with references to the Gutenberg TradeMark, then you need to follow some special rules. 

Further on in the document it says:

          DISTRIBUTION UNDER "PROJECT GUTENBERG-tm"

You may distribute copies of this etext electronically, or by
disk, book or any other medium if you either delete this
"Small Print!" and all other references to Project Gutenberg,

This clarifies what you need to do if you want to distribute the ebook without any restrictions - basically you delete the 'license' and the Gutenberg trademarks.

It then goes on to elaborate the conditions you must follow if you do want to distribute the text with the Gutenberg trademarks:  

or:
[1] Only give exact copies of it ...
[2] Honor the etext refund and replacement provisions of this
"Small Print!" statement.
[3] Pay a trademark license fee to the Project of 20% of the
net profits ...

Note: I am not a lawyer, and this is not a legal opinion.