The code in media/audio/audio_manager_base.cc says:

// If the caller supplied an empty device id to select the default device,
// we fetch the actual device id of the default device so that the lookup
// will find the correct device regardless of whether it was opened as
// "default" or via the specific id.
// NOTE: Implementations that don't yet support opening non-default output
// devices may return an empty string from GetDefaultOutputDeviceID().

See 
https://cs.chromium.org/chromium/src/media/audio/audio_manager_base.cc?q=audio_manager_base.cc&sq=package:chromium&dr&l=243

This means that if the default device changes while audio is being played, output will continue on the old default device.

CrasAudioManager is not originally implemented to behave this way.
When the default device is selected and it is playing audio, it the default changes, then audio switches to the new default device.
When changes were initially made to CrasAudioManager to support group IDs (see bug 636300), CrasAudioManager's behavior changed to what the comment above suggests is the intended behavior for an AudioManager.

This was considered a bug (see  bug 748267 ) and the changes were reverted. Support for groupID will be relanded, but without changing the behavior of the default device.

It is not 100% clear if the current behavior in CrOS is actually correct, since it differs from other platforms.
The purpose of this bug is to decide on this matter and update the code if necessary.