Issue occurs when using Chrome on a windows 10 device. According to BrainPop, the default for videos is to use flash and if its not present to use HTML5. User can switch to another browser or perform the steps below.
In chrome, go to BrainPop and play a video. If no audio is heard click on the icon to the left of the url on the menu bar that as a lock and says "secure"
You should see FLASH with a drop down arrow to the right. Click on the arrow and select ALWAYS ALLOW ON THIS SITE
The page should refresh else force it to and the sound will play
Please note, if the browser is reset to default or ccleaner clears all settings, the user might need to perform these steps again.