Where can I get full audio cockpit recordings of all four flights involved in the 9/11 attacks?
The short answer: you can't. Federal law prohibits the release of CVR audio, though transcribed excerpts (if they are relevant to the investigation) are made publicly available once the accident report is released.

See 49 USC 1114(c):


  Cockpit Recordings and Transcripts.—(1) The Board may not disclose
  publicly any part of a cockpit voice or video recorder recording or
  transcript of oral communications by and between flight crew members
  and ground stations related to an accident or incident investigated by
  the Board. However, the Board shall make public any part of a
  transcript or any written depiction of visual information the Board
  decides is relevant to the accident or incident— (A) if the Board
  holds a public hearing on the accident or incident, at the time of the
  hearing; or (B) if the Board does not hold a public hearing, at the
  time a majority of the other factual reports on the accident or
  incident are placed in the public docket.