Why does the NTSB release transcripts of Cockpit Voice Recordings, but not the actual recordings?
From NTSB site, regarding FOIA requests:


  "Cockpit Voice Recorder (CVR) tapes. Title 49 U.S.C. § 1114(c)
  prohibits the release of any CVR tape. However, the NTSB may release a
  CVR transcript (edited or unedited), in accordance with 49 U.S.C. §
  1114(c)(1)."


Transcripts may be disclosed, but not 'tapes'. Which makes sense, as transcripts may be edited to remove parts that are not meaningful for the investigation, but can be  unnecessary intrusions into private life.

In addition, when the NTSB is participating to foreign investigations:


  Records or information relating to the NTSB's participation in foreign
  aircraft accident investigations. 49 U.S.C. § 1114(f) prohibits the
  release of this information before the country conducting the
  investigation releases its report or 2 years following the accident,
  whichever occurs first


See 49 U.S. Code § 1114 - Disclosure, availability, and use of information