Why isn't Hillary Clinton releasing the transcripts of her paid Goldman Sachs (Wall Street) speeches? If she wins the Democratic nomination, she risks facing these questions again in the general election campaign, especially if there is something damning in those speeches.
I doubt that she knows of anything in particular that is problematic. The issue is that it is likely that if she released the transcripts, she would have said something, sometime that Republicans could criticize.
Look at what happened in 2012 when a recording of Romney talking at a fundraiser was released. The 47% statement became a meme for the Obama campaign.
Or look at what happened with her emails. She sent an email that Republicans use to accuse her of suborning the misuse of confidential information.
Or her superpredator comments from 1994.
Talking is dangerous for a politician. She doesn't have to have been plotting a crime. She could have simply said something that sounds bad. For example, if she commiserates with her listeners about something, she could be accused of putting rich people problems ahead of poor people problems.