First Potential Problem - Your Sound System is Faulty
It may seem a rather obvious solution but have you checked the sound is not muted and is not at zero volume?
Many support queries we’ve received on this subject has been caused by the volume being switched off on the device (or an attempt to view on a device that has no ability to play sound).
Play some music or other sounds to prove that your sound system is working.
Second Potential Problem - Your Web Browser is Faulty
Even If you can view videos from other sites it does not prove that your browser is working properly.
Browsers are notorious for developing faults. This is because they support third-party “plug ins” that often have bugs.
You may find that some sites (such as YouTube) work with your browser. That’s because many older sites use an older method of displaying video that only works when you install the Flash plug-in. Our site uses the more modern HTML5 video method which doesn’t need any third-party plug-ins to be installed to work. The browser itself is responsible for playing the video. HTML5 is vastly superior to Flash and enables several very useful features (such as vari-speed play).
If Flash has been added to your browser (or a different non-Flash video plug-in) you may be able view older video sites (that still use Flash) but still find problems with ours. This is because Flash is masking the browsers inability to play HTML5 video (when you visit an old-fashioned Flash site).
If this is the case the easiest fix is often to simply use a different supported browser (for example, if Chrome doesn’t work try Internet Explorer, Safari or Firefox).