Audio may unexpectedly revert back to playing through your computer's internal speakers (or the sound out port) when a Bluetooth audio device, such as headphones, gets disconnected. Some examples of how a Bluetooth device can get disconnected include dead batteries, being powered off, or going out of range.
Even after the audio reverts to your computer's internal speakers, the Sound preference pane will still show Bluetoooth as the selected sound output source. When opening the Sound preference pane, a "Bluetooth Audio Failed" message will appear. You should simply dismiss the message and select "Internal Speakers" as the output source. This will get rid of the "Audio Failed" message and allow you to avoid further confusion until the next time you use the Bluetooth audio device.