I have a 2021 iPad pro 11 and am seeing an consistant problem, sometimes temporarily resolved by a restart. On some website videos I cannot get any of the desired audio. This model iPad has no mute button or switch.
The iPad does have a Silent Mode. The bell icon can be seen in the Control Panel (pull down from the top right corner). If not there, you can turn it on/off in Settings > Control Panel.
Silent mode normally only affects alarms and alerts, however some poorly written apps and sites have been known to be silenced with this control.
The more likely cause is that the playback controls on the site or app has it's own volume/mute icon. This will override the volume settings on the iPad. It usually looks like a speaker icon.