Mute iPad when connected to external monitor

I hooked up my iPad to my Dell 27 inch monitor (which has speakers) and play a video, via USB-C and HDMI cable.

When the iPad is hooked up to the monitor, I have no ability to control the sound.

I want to mute the sound, but cannot. I want to play a silent video, but cannot.

When I swipe from top right to get control center, the AUDIO tool changes to a MONITOR logo.

How do I mute the sound on my iPad, when it is connected to an external monitor.

Thanks.