If your sessions have sound and your videos don't, the first thing to check is that sound is turned on on your device. The on/silent switch on the left hand-side of your device should be switched on.
Be sure to also check the volume controls on your device, which are also available on the left hand-side of your device.
We've also heard that turning off Bluetooth (if it's on) while playing videos might help, so give that a try as well!
If that doesn't do the trick, email us at firstname.lastname@example.org and let us know!