Check and make sure that the RCA jack is properly plugged in at the back panel (generally, light green socket). Make sure that the "Volume" is turned up, both on the computer volume control as well as on the right speaker volume control. The RCA plug tends to dislodge itself when the computer is moved. Check for operation.
See if the speakers are "enabled" by checking Device Manager.
The get into Device Manager, start Control Panel ---> Administrative Tools ---> Computer Management ---> Device Manager