Im pretty new to the custom keyboard world. Are there sound differences (if any) when using a plastic case vs aluminium?
For me there was a difference in sound. Aluminium base plates are louder and more high-pitched with the typing experience, but generally feel better (more premium) - it's mostly down to the down-press so the harder you hit the key, the more is the clang of the sound. It's not obscenely loud, but noticeable.
There's also the profile of the keyboard to consider, where a high-profile keyboard will muffle some of the down-press (and you can add foam underneath the base to help that further).