Currently, for editing an existing media resource (like audio or video), you must select the control completely using the mouse or the keyboard as you can see in the following screenshot and then click on the Media button in the Atto editor (it will appear slightly selected). At least this is the only way I've found for selecting it with Chrome/Firefox in Ubuntu 18.04 and Windows 10 (through BrowserStack).

This is not easy so it would be good to review it and find a more intuitive way, like double click on it.