If a vertebrate had a paired fin (specifically a sarcopterygian-like fin) in the correct place, would it be plausible for the fin to adapt into a structure analogous to the middle and outer ears of tetrapods? The vertebrate has no hearing parts besides the inner ear, and would be transitioning to land as this structure evolves. It also has many fins along its body
Likely not, at least with hearing as we think of it.
Darwin's mechanism for evolution and natural selection requires gradual changes, where each one is slightly better than the last. Any change that does not produce immediate results should be filtered out by Natural Selection.
A fin would start with no apparatus analogous to an ear, and I can't think of any sequence of small changes (each improving on the previous body design) that would give a fin an entirely new function.
That said, we do have limited ability to hear through our bones. I can't find it upon googling, but my band director once showed me a silent tuning fork, where it would vibrate without making noise, but if you placed the end on your elbow and touched your eardrum, you could hear the tone. Another example would be placing the vibrating tines of a fork between your teeth and "hearing" the sound transmitted through your bones.
You could probably develop some similar structure to this where the creature "hears" through vibrations conducted up through its fin (especially if that fin is used to walk on later in the evolutionary sequence). This would be much easier to create via a Darwinian evolutionary path.
Now that I think about it, that is a very similar function to how snakes can "hear" vibrations in the ground. You might look into that as well for inspiration.
I hope that helps you!