First, determine if the problem is with the individual speaker by swapping the wire at the back of the satellites. Also, check the speaker wire itself. Swap the wire for the speaker that is working for the one that is dead. If it's the speaker or wire that doesn't work, contact customer service at 800-554-7724.
If it's a channel is out, make sure the plug is firmly seated into the soundcard. Also, you can test the speakers with another device (you can plug the audio cable into any headphone output jack of devices such as a portable CD player, walkman, or mp3 player) to determine if the problem is with the subwoofer or the soundcard. If a channel is still out when using the other device as a source, please contact technical support at firstname.lastname@example.org. If the speakers work with the other device, you may need to reinstall your soundcard drivers—contact your soundcard manufacturer for further assistance.