Why are only 2 out of 4 speakers working on my iPad 10 gen?

Don’t be fooled by your iPad Air seemingly having four speaker grilles. Your iPad Air has only two speakers - not four as found in models of iPad Pro.

Recent models of iPad and iPad Air have one speaker at top and bottom - such that what viewed in Landscape orientation, you will have a left/right stereo pair.

You can see and compare iPad specifications here:

https://www.apple.com/ipad/compare/?modelList=ipad-10th-gen,ipad-air-11-m2,ipad-pro-11-4th-gen

The iPad 10th gen except the Pro models only have two (stereo)-speakers. But they also have two microphones, so they might be in these two "speaker" grills, especially because they are a bit smaller.