Judging from the source (viewable with Chrome extension source viewer [1]), it's POSTing the IPA text to https://www.ipaaudio.click/audio as the `ipa` key of a JSON object, the server does the heavy lifting and sends back an audio buffer, which the extension sends to a WebAudio context.

It'd be neat to know what exactly the server is doing, though.

[1]: https://chrome.google.com/webstore/detail/chrome-extension-s...