Are all the sounds to be created by the developers, or can non copyrighted sounds could be used?
It needs to be created.
Correct, you need to create your sounds yourself
Or use sound generators like bfxr / sfxr
And something like Chrome Music Lab is also allowed?
Yes, Chrome Music Lab is also allowed. Exception to this is the piano roll thing where it has full songs by default. You can of course use this one if you change it up entirely.
Maybe this is a dumb question, but on creating sounds, does this rule apply to instrumentation provided in a program like Cakewalk?
If they are purely samples / plugins then it's allowed.
