In order to prevent incidents related with information leakage, many enterprises and organizations have installed an air-gapped system. The system is used for separating their own network from a public network such as the Internet. However, researchers have demonstrated possibilities that the air-gapped system can be inactivated by attackers, especially about their advanced attacks with various covert channels. In this paper, we analyzed how much the information could be leaked via the covert channel. We conducted experiments about data communication between a speaker and a microphone which are regarded as a conventional acoustic covert channel. At the same time, we also had expanded the attack scenario into an environment without any microphone. That is, we tested whether the critical information could be leaked and transferred via two loud-speakers as a limited environment where the air-gapped system. Finally, it is shown that the speaker based covert network can be effectively expanded to centrally controlled embedded loudspeakers which have not been considered in a conventional acoustic covert channel.