Ian Fitzpatrick and Philip Stott, former members of the Scottish Socialist Party (SSP), both said they were certain it was someone else speaking on the video posted on the News of the World website.
However the jury, which must decide whether the politician and his wife Gail lied in a defamation hearing against the newspaper, also heard how Sheridan himself admitted the voice could be his during a TV interview. Four prosecution witnesses had previously told the High Court in Glasgow that they knew it was his voice.
Mr Fitzpatrick, 36, of Motherwell, who is now involved with Sheridan's Solidarity party, said he had watched the video when it was released online.
Sheridan asked him: "We have heard four Crown witnesses state that is me in that video. What do you say?"
Mr Fitzpatrick replied: "Absolutely not. In so many ways, it is just not you. Knowing the length of time I have known you, I know for a fact it is not you. You do not behave like that, swear like that, and speak slang like that."
He said he was "absolutely sure" the voice was not Sheridan's, before adding that with two young children, there was no way he would risk going to prison by lying.
Mr Fitzpatrick said the "voice" swore repeatedly, and there were long pauses. "He (Sheridan] is somebody who always knows what he is saying and says it in a very eloquent way," he added.
Mr Stott, 46, of Dundee, said he and Sheridan had collaborated politically for about 25 years, and had been founding members of the SSP and Solidarity.
He told Sheridan: "I could not see you in the video and the voice I heard certainly was not yours. I am absolutely certain."
He added: "I would probably describe it at best as a poor caricature."
Sheridan was interviewed by Jon Snow on Channel 4 in 2006, just days after the News of the World had released the video. Prosecutor Alex Prentice, QC, played a video of the interview.
In the Channel 4 interview, Sheridan said: "I categorically deny that I was involved in that tape.They may have my voice and may have inserted my voice in a tape, but there again, who framed Roger Rabbit?"
He had also said: "I certainly was at no meeting in relation to the creation of this particular audio tape, and at the end of the day, what they have done is they have concocted a spliced version of a tape which may include my voice, but certainly includes somebody else's voice."
Sheridan, 46, won a defamation action in 2006 against the News of the World over allegations it had printed about his private life. He and wife Gail, 46, deny giving perjured evidence.
Their trial has been told that George McNeilage, a former friend of Sheridan, came forward with the video and sold it to the News of the World for 200,000. He said it had been recorded covertly two years earlier, and that in it Sheridan, who could be heard but not be seen, admitted visiting a swingers' club.
Both of yesterday's witnesses criticised police handling of the case. Mr Fitzpatrick said police had put cards through his door, trying to contact him, and he thought it "bordering on the extreme, why they were going to these lengths".
Mr Stott said: "I do not believe Lothian and Borders Police conducted an even-handed approach. I would describe it as a vendetta."
The trial continues.