[symstym]

> is Web Audio a mess mostly because it's an attempt to expose the features of the twenty-odd different OS audio backends

That is a good guess, but no. The main features of the Web Audio API (built-in nodes, etc.) are not backed by any kind of OS-level backend, it's all implemented in software in the browser. The spec design was based on what someone thought were useful units of audio processing. It's not a wrapping/adaptation of some pre-existing functionality.