Since when has "Freedom of expression" especially in regards to non-expression applied to Court Rooms? If it did than all Perjury laws would be in violation, as false testimony to get a conviction could be deemed "Protected Speech".... Worst case: Someone innocent gets put on Death Row because the testimony that could have cleared them wasn't given because a witness exercised "freedom of expression". I'd say this "worst case scenario" would totally undermine the purpose of the criminal justice system.
So no, you do NOT have the right to remain silent in a courtroom. You must testify, and before you do you must swear a legally binding oath to tell the truth. The issue isn't witness rights, it's defendant and plaintiff rights. Defendants has the right to call witnesses to exonerate themselves. If a defendant isn't particularly popular than witnesses may not tell the whole truth if they had the "freedom of expression" to do so. Plaintiff's have the right to call witnesses to make their case against a Defendant, if the witnesses have "Freedom of Expression" to falsify testimony than a defendant could get away with some crime (ie: vandalism, shoplifting, whatever) just because a witness exercised their "Freedom of Expression".