In the Observer pattern, the Subject still needs to hold references to the Observers so they can be notified of any events they are interested in. If you don't want the Soundcard to hold a reference to the Computer, you're still going to need them to be connected in some way. A message queue is one way to do that, where the Soundcard publishes to a topic that the Computer is watching. This is the Publish-Subscribe pattern.
Practice only makes habit, only perfect practice makes perfect.—every music teacher ever
Practice mindfully by doing the right things and doing things right.— Junilu