Chrome OS may have supported Linux applications since 2018, but it debuted with quite a few limitations. Though GPU acceleration finally started to land for some Chromebooks in the middle of last year, the Linux container still hasn't been able to capture audio input for things like microphones. Or, at least, it couldn't until the feature silently landed with Chrome OS 79.
First spotted by long-time Chromebook expert Kevin Tofel at About Chromebooks, the new feature isn't enabled by default, or even hidden behind an easily-accessed flag; it's too early for that. While a flag may debut with a later release, to enable the feature now, you'll have to muck about with some commands in "Termina," the Linux system that handles Chrome OS' Linux VMs/containers. The following sequence of commands/instructions is required:
# Use Ctrl-Alt-T to open crosh crosh> vmc stop termina crosh> vmc start termina --enable-audio-capture
After running these commands, you can safely close the crosh terminal's tab and ignore the warning it gives you. These commands kill Termina, then re-launch it with a new option to enable audio capture, and that should be enough for subsequently spawned Linux apps to pull sound from microphones, including the built-in ones.
The feature has been tested and works with applications like Audacity and should work with any audio input device recognized by Chrome OS, including external devices like dongles or USB microphones. The only real drawback right now is that you might have to sort through a handful of audio input devices before you find the correct one, and you might have to perform these steps again if you restart your Chromebook.