This makes setting up sounds for random pitch/volume faster, as you
don't have to change the mode from Random (Avoid Repeats) to Random
anymore if you only care about randomizing pitch/volume but want
to prevent a warning message from appearing on every playback.
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
