The Logitech Z-640 is a surround sound speaker system designed for use with Windows-based computer systems. With the added speaker set-up and increased audio performance, it is more likely to run into playback issues, ranging from some speakers not playing audio to the equipment failing to produce surround sound. Regardless of the issue, it is possible to fix whatever problem you are running into.
Check the cable connection running from the "Line-Out" port on the computer and the subwoofer of the Logitech Z-640. If the cable is not inserted completely into both ports, the audio signal is not going to reach the subwoofer.
Examine the wires running from the satellite speakers to the Logitech Z-640 subwoofer. These speakers must be inserted into the given wire openings on the rear of the subwoofer. If not, some of the satellite speakers are not going to produce the received audio.
Click the audio icon in the lower right corner of the desktop, then deselect the "Mute" option. This ensures the speakers are receiving audio.
Click "Start," "Control Panel" and then select the "Sound" icon. Click "Settings," then set it to a surround sound set-up (instead of the given stereo option). This ensures all speakers connected receive the correct sound.