In the words of the U.S. Government Printing Office, a hearing is "a meeting or session of a Senate, House, Joint, or Special Committee of Congress, usually open to the public, to obtain information and opinions on proposed legislation, conduct an investigation, or evaluate/oversee the activities of a government department or the implementation of a Federal law. In addition, hearings may also be purely exploratory in nature, providing testimony and data about topics of current interest."
There is no single online database covering all U.S. Congressional hearings.
Instead, we have access to separate databases with coverage of different time periods and formats:
Leyburn Library's U.S. Goverment Depository Collection includes thousands of paper copies of Congressional committee hearing transcripts, most of them published within the past twenty years. Here is a partial list, consisting of the most recent 32,000.
You can search for specific titles or subjects in the library catalog.