Is it mostly something you notice when you have them side by side in a silent environment and they are on the hard surface like table. Using them on your lap "muffles" the sounds quite a bit.
There was some parts of the keyboard in Pro than made very hollow sound, while Air was solid.
Keyboard "hollowness" was worse in 1.4GHz model, more powerful 13" models were better probably due to more hardware under the keyboard (like second fan and more heatpipes).
Also the same with the trackpads. In Pro it is hollow sounding when you keep clicking it repeadately, kind of a "clunk clunk" type of sound while in Air it is mostly just you feel the click with your fingers and hear only silent very solid kind of click sound.
The best I can describe the difference here is similar like with the cars, when you compare 20 year old Toyota Corolla vs new Mercedes Benz by slamming the door closed. The latter one is very solid feeling / sounding while the other is quite the opposite. But they both take you where you need to go, so no difference in practise.
Also there was same difference when you knock the bottom plate, Pro sounds hollow and Air very solid. I guess Air insides are packed more tightly making the difference in sounds I describe here.
Looking at iFixit teardowns between the Pro and Air in the past couple of years, seems there is no change in internal structure, so I doubt what I described has changed much recently.
I don't know if something has changed with M1 between Air and Pro but with the olders ones Air seems to get better points in speaker quality in Notebookcheck reviews. Also I could hear that with Air and Pro I had side by side playing the same audio files and muting each one briefly to listen the difference, it was pretty obvious that way. Later on when I had only my Pro I gotten use to it, it was pretty balanced but it always sounded very tinny lacking bass completely, never felt that with Air when I only used it.