A communication system for broadcasting audio messages in multicast mode, comprising: a Voice over IP (VoIP) audio communication network, which network comprises: at least one transmit/receive client unit and one or more receiver units for receiving a multicast audio message sent by the at least one transmit/receive client unit and each of which receiving units having means for broadcasting the audio message; an Ethernet network for connecting the said at least one transmit/receive client unit and one or more receiver units; at least one exchange for connection of the transmit/receive client unit/s and the receiver unit via an Ethernet network, each transmit/receive client unit and each receiver unit being uniquely identified by an IP address; characterized by the exchange comprising a PBX; a multicast communication server unit being provided which is connected to the PBX exchange for connecting to the transmit/receive client units and to the receiver units, which multicast communication server unit comprises a multicast streamer, a receive/transmit terminal connected to the PBX for communication with the transmit/receive client unit and to the one or more receiver units for transmitting to the means for broadcasting the multicast message of each of the said receiver units in multicast mode; the multicast communication server unit receives from the transmit/receive client unit a control containing the information required for initiating the multicast session comprising recipient addresses of the receiver units and generates a session announcement message to be sent over the network enabling the receiver units to receive and broadcast a multicast message stream generated by the multicast message streamer. a Voice over IP (VoIP) audio communication network, which network comprises: at least one transmit/receive client unit and one or more receiver units for receiving a multicast audio message sent by the at least one transmit/receive client unit and each of which receiving units having means for broadcasting the audio message; an Ethernet network for connecting the said at least one transmit/receive client unit and one or more receiver units; at least one exchange for connection of the transmit/receive client unit/s and the receiver unit via an Ethernet network, each transmit/receive client unit and each receiver unit being uniquely identified by an IP address; at least one transmit/receive client unit and one or more receiver units for receiving a multicast audio message sent by the at least one transmit/receive client unit and each of which receiving units having means for broadcasting the audio message; an Ethernet network for connecting the said at least one transmit/receive client unit and one or more receiver units; at least one exchange for connection of the transmit/receive client unit/s and the receiver unit via an Ethernet network, each transmit/receive client unit and each receiver unit being uniquely identified by an IP address; characterized by the exchange comprising a PBX; a multicast communication server unit being provided which is connected to the PBX exchange for connecting to the transmit/receive client units and to the receiver units, which multicast communication server unit comprises a multicast streamer, a receive/transmit terminal connected to the PBX for communication with the transmit/receive client unit and to the one or more receiver units for transmitting to the means for broadcasting the multicast message of each of the said receiver units in multicast mode; the multicast communication server unit receives from the transmit/receive client unit a control containing the information required for initiating the multicast session comprising recipient addresses of the receiver units and generates a session announcement message to be sent over the network enabling the receiver units to receive and broadcast a multicast message stream generated by the multicast message streamer.