
In essence you are using the source and the digital converter in your computer. Your computer speakers are running off of the amp in your computer also. Loudspeakers need more power to run efficiently without running out of current. So the normal way is to go into an external amp, receiver, or something and hook your speakers up to that.
You can get a decent receiver cheap, at the Salvation Army with speakers ,that should do what you want, as long as you check the inputs to the receiver before you bring it home.
The receiver should also have a headphone out.