Roll20 features a built-in video and voice chat feature (powered by Tokbox) that allows you to communicate with other players.

Note: If you aren't having much luck with the built-in video/voice chat, you might also try running your game in a Google+ Hangout, which offers an alternative way of having video/voice chat in your game.

Enabling and Disabling Video & Voice Chat

Go to the My Settings tab (the gear icon) in the Sidebar (on the top-right side of the screen). Scroll down to the bottom of the tab to find the Video + Voice heading. There will be three related drop-down menus underneath. They are:

I want to Broadcast to others: this controls whether or not you will broadcast video and voice, voice only, or nothing to other players in the game.

I want to Receive from others: this controls whether or not you will receive video and voice, voice only, or nothing from other players, if they are broadcasting at all.

Video/Player Avatar Size: This controls the size of the player avatars that are shown in the Player Zone in the bottom-left side of the screen. Note that if you choose "Names Only" you will not receive or send any video or voice broadcasts, regardless of the other settings above.

Managing Volume

While video/voice is enabled in Roll20, hover your mouse on the bottom right corner of a player's portrait in the Player Zone. This action will pop up a volume slider that you can use to fine tune a player's mic volume to be heard properly. It's important to note that a slider exists on your own portrait and tuning your personal mic volume affects how other players in Roll20 hear you.

Switching Devices

To switch between multiple microphone and webcam devices, once again hover your mouse on the bottom right corner of your avatar in the Player Zone. Next to the volume slider that pops up is a wrench button. Clicking on this button brings up a new window where you can specify which devices you wish to use for chat.