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 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 verify and compare iPad specifications here:
https://www.apple.com/ipad/compare/?modelList=ipad-pro-12-9-6th-gen,ipad-air-5th-gen,ipad-air-4th-gen
As a point of interest, older models of iPad Air had two speakers on the bottom edge of the iPad - one placed either side of the Lightning port. For these models, with the iPad positioned in Landscape orientation, both speakers were to one side of the screen.
If the audio level differs substantially, check the audio balance settings:
Settings > Accessibility > Audio/Visual > Balance
Should you suspect that you have a hardware issue, you would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and should a fault be confirmed, you can explore your available options:
Genius Bar Reservation and Apple Support Options - Apple