If you are the person streaming a video, the stream audio is muted by default in your Evercast app. Otherwise you would hear audio coming from both your video stream and the Evercast app at the same time.
If you would like to hear the stream audio, simply un-mute the stream.
On the Evercast desktop app, hover over the stream thumbnail and select the down arrow. Then select Turn on audio.
On Evercast for Chrome, go to the stream thumbnail and click on the "three dots" button. Select Sound on from the menu.