As the instructions indicated, swipe up and then take the screen capture.
iGigBook Sheet Music Manager App for iPad and Android Tablets
The screen you posted is the result of swiping right. I'm asking you to swipe up to display the control center. Here's a link to Apple's documentation about it which will help you identify it. https://support.apple.com/en-us/HT202769
The first icon(open lock in the circle) of the last block of 4 icons at the bottom on my iPad is a Bell not an Open Lock. The Bell turns "Silent Mode" on and off but it looks like your setup has swapped the functionality to show Lock/Unlock. Your problem is that "Silent Mode" is on, which is why you don't hear any sound from the metronome. You will need to check with Apple resources on how to get the "Silent Mode" icon to appear where the "Lock/Unlock" icon is.