As salaamu 'alaykum
If the Imam ascends the mimbar and greets the audience with the salaam then the audience should respond. This would not be considered frivolous discourse and would not violate the silence required for listening because the actual khutbah has not yet begun. Therefore, until then, the audience can recite praises of Allaah and duaa’, and also repeat after the mu’athin who calls for prayer, which is timed after the Imam has actually greeted the audience and has taken his place on the mimbar.
Sheikh Muhammed Salih Al-Munajjid