Same issue here. I've searched convos about this a couple times now and mostly every search yields conversations about cleaning out earpods. But my AirPods are clearly functioning well. An initial computer connection is even fine. However, I can walk away and whether I get to a disconnecting distance or not, chances are when I get back to my computer and press 'play' on whatever, the volume is so low I can't discern a thing, all while my computer's volume is completely maxed out.
The only thing that has changed the output volume has been connecting to something else, usually my phone, then reconnecting. Magically, there is now a normal volume level.
You should try it and see if this works for you. While it may give you satisfactory volume again, the mystery remains as to why the volume mysteriously got lower in the first place.