If you look in the /examples directory there are some basic examples, and one advanced one called wvr_thames.ino.
Most of the basic Arduino functionality can be accessed through the WVR class, so you have WVR.play(), etc.. You also have access to the midi hook, which you can see how that works in the wvr_thames.ino file.
You can also always fork, clone, or otherwise modify the WVR source library, like you said its all in the wvr/src directory, but be warned, the firmware is very complex, and it is easy to create bugs. In the case of a bug, the WVR can be bricked, you wont find its WiFI to upload a fix, and so you absolutely need a usb-ftdi module, as described in the documentation, so you can flash and receive logs over USB.
The MCU on these usb-ftdi modules is included on most ESP32 dev boards, I did not put it on the WVR because it costs 2 pins, adds cost, and takes up a lot of space, it is a fairly large MCU with several peripheral parts. Luckily the modules are easy to find, and very cheap (around $10).
I am always here to support anyone hacking on custom firmwares, don't be shy to ask questions here, I always try to respond within a day or 2.