In order to join public discussion channels you need first to install discord or connect to it using a web browser. Then you need to create a discord account for logging into discord. Once there you need to join the ntop server clicking on this link
In particular for voice channels and joining public discussions, you need to click on the “public” channel under the voice channel as shown in the picture below
Once there you are in the public voice channel so if you speak everyone can hear you.
At the bottom of the left discord sidebar you can see the above image once you are connected to the channel. You can disconnect clicking on the X icon, or share your webcam or your screen clicking on the icon below. At the bottom of the screen you can find some icons that allow you to change your multimedia settings, mute etc.