Hi @snipesc,
Sounds like an interesting project! Looks like you've already gotten some good advice. I would start off by looking at it from the big picture and thinking about what you want to accomplish with this study specifically, then make design decisions from there. Things like: Do you need a 2D or 3D model? Do you have access to surface files for these structures? Do you have access to material properties for these parts and know what FEA material model to use? How will you connect all of these parts together to accurately represent the anatomy of a bat? And then how will you simulate the vibration and how will you measure the echo?
Since you are interested in the echo mainly, then I would focus on that aspect of the problem and make assumptions regarding the materials and vibration transmission. FEA gets infinitely more complicated when you start to cross between things like solids and fluids (FSI - fluid solid interaction). So maybe do a simplified 2D analysis to start where you just study the sound wave created from the vibration and the resulting echo?
I would also make sure to triple check all units and hand calculations as you go (120 db does not equal 1N).
Does your school have an engineering department? I would highly suggest partnering up with someone within engineering who has experience within FEA. Otherwise, this will be quite difficult for somebody to pick up and model something with meaningful results.