Since you only want one extra room you can connect a wireless audio transmitter to your PC. It will need to use the same connection as your PC speakers since you can't run two audio devices at the same time.
There are a lot of different ones. You don't need anything fancy since your speakers don't warrant it. You will need a way to control the volume in that room so the receiver will need a volume control if the speakers don't have a convenient one built in.