First, check that the MUTE button on the side of the device is not set to silent mode. On certain models of iPads, this has been found to mute web-based videos even when other apps have audio playing.
If that does not solve the issue, please explore the following suggestion:
When you watch a web based video on the iPad, it plays with a video controller embedded on the page. This has its own volume control, which is in the player's control bar (see image below):
However, if you choose to expand the video to full screen mode, the internal iPad video player takes control.
This player has a separate, and overriding volume control (see image below):
So, in short, there can be a circumstance where the iPad volume control is low or muted, but the embedded control appears to be up. The internal video player audio control even overrides the manual volume buttons.
The way to check is to play the web video, choose to expand to full screen and the check the level of the audio controller in the player.