This week's book giveaway is in the NodeJS forum. We're giving away four copies of Serverless Applications with Node.js and have Slobodan Stojanovic & Aleksandar Simovic on-line! See this thread for details.
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.