Surprise! Chrome OS 79 introduced support for microphone audio capture in Linux apps on Chromebooks! It’s in the Stable Channel right now; here’s how to enable it.
microphone
Don’t expect to record audio or have Skype video chats using Linux for Chromebooks any time soon
Sound support is still in the works for Linux apps on Chromebooks, but the first function will simply be that: Sound playback. Audio recording is slated to follow and camera access — needed for Skype video — isn’t even on the roadmap yet.