System Audio capturing broken recording on ChromeOS 54
Due to a current bug in ChromeOS (affecting ChromeOS version 54 and 55), no system audio is captured with desktop recording.
If you can, use tab recording (select Tab -> Record Tab) in the meanwhile until this is fixed. Tab audio capturing still works fine.
This Bug has been fixed in ChromeOS 56 or newer, so it will work again after your ChromeOS device has been updated.