There is no audio when listening on Mozilla Firefox's browser.
The RealPlayer functionality, Downloading & Recording, has compatibility issues with Flash Player 11.3. If RealPlayer is installed, Flash Player content does not appear. YouTube provides an error message when you try to watch a video.
As a workaround, do the following:
1. Start RealPlayer.
2. Click the RealPlayer icon in the upper-left corner and select Preferences.
3. Select Download & Recording in the left menu and deselect Enable Web Download & Recording for These Installed Browsers option.
4. Click OK and restart Mozilla Firefox.
Help information originated from Adobe's website: