Where can I watch transcripts of criminal trial in the United States?

Here

Try www dot pacer dot com

The website says “There is a fee to access court records once logged into PACER.”

It seems that everybody is charging for their work now days

You can view a lot of PACER records for free before the (low) fee kicks in.

“Users are billed on a quarterly basis. Fees are waived for anyone accruing less than $30 in a quarter. Learn more about how PACER fees work. ”

At ten cents a page that is 300 pages free per quarter. I get an email every quarter saying “Balance due: $0.00. In accordance with the Electronic Public Access fee schedule, the fees for this quarter have been waived.”.

They do not have transcripts of the court proceedings. They have the written records submitted by both sides.

Note that PACER is for federal courts. Most crimes are prosecuted by the states in county courts. Whether they have records available on line is going to vary from county to county.