First of all, this is a question specifically asking about the September 11th, 2001 attacks. It is not focused on NTSB or other accident investigations in other countries. If you think it is, please explain your reason. I have always been curious to know how people reacted to that terrible day, and if I remember correctly, there was a thirty-one-minute recording from flight 93 that started with someone saying there was a bomb on board up until the crash. I guess the main reason I am so focused on sound is because of something I am going to explain to you all. I am a totally-blind user who uses screen reader technology and accessible web sites to brows. Since I cannot obtain visual information and expressions, my next source of information is how things sound. It is frustrating to not have this information. Someone could tell you that this or that happened, and it looked horrific, but if you can't see it, how do you know? Maybe some people would say that it was a blessing that you didn't have to see such things, but still, I feel like I'm missing out on something. Sounds of horror, of grief, all of those can be just as traumatising to hear as one would see an image. I have spent numerous hours on YouTube, trying to find stuff to add to my audio memorabilia of 9/11, because as we move through history, I want people of the future to know how everything played out from beginning to end, what people endured, how many people were killed, hurt, and willing to risk their lives to help rid the bad guys. Most importantly, I want them to experience all of this without the ability to see what's going on. If you can hear the recordings, you should be able to make pictures based on all of your senses, and you'll be able to make appropriate emotional responses. Also, a friend of mine inspired me to do all of this because she had a former acquaintance of hers, who is no longer her friend tell her that 9/11 never happened, that it was all a conspiracy. So I vowed to help her out by obtaining as much proof as I could by making a file containing as many audio recordings detailing the events of that day. I have heard that the NTSB is exempt from releasing these audio recordings even with the FOIA, but I believe that if you can approach an attorney, you can circumvent this exemption by claiming that such recordings must be available for historic reasons. I don't know if they will release it twenty-five years later, fifty years, seventy-five, or a hundred years from now. And yes, there has been evidence of CVR recordings being leaked into the public, which is like finding a rare piece of jewellery since they are scarce.
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.