Is it protected free speech to call people "liars and frauds who should be in jail"?

It could be, as long as the claim is well substantiated. Otherwise it is libel. But the claim appears to be extreme in the episode mentioned below -- where a professor was demoted for calling some government officials that:

"The university's independent report into the incident, part of which has been seen by HES, concluded that Dr Mees at a conference in August last year had effectively vilified then director of public transport Jim Betts and other government officers by making "derogatory and insulting" remarks. Dr Mees allegedly accused the officials of being "liars and frauds who should be in jail" over the preparation of report on public transport.

The report, by Monash University law professor Michael King, acknowledged some comments that Dr Mees had made valid points over the governments claimed that consultants report was independent when in fact parts of the report had been copied verbatim from as government memo. But the report found that Dr Mees went too far in his remarks. "Academics are entitled to be forthright in their views. But it isn't their role to make allegations of personal misconduct or criminal conduct in a public forum," the report said.

Source