1 set of speaker jacks not working, grounding out?

These are analog speakers and do not need a driver. First, make sure they're plugged into the speaker/headphone jack and not the microphone jack. Second, make sure the computer's sound output is enabled and set the headphone/speaker jack. Third, verify that the sound output is working by plugging in a headphone set or a different pair of speakers. If the computer sound checks out, the speaker cable is bad or the power supply is not working.

This more about what you are connecting the speakers to.
The headphone jack is essentially a switch that mechaniccally opens the ground connection to the speakers. It is a spring that is opened by the headphone jack pushing the spring contact open. What often happens is the headphone plug gets pulled at an angle and forces the clip to be open permanently.
Sometimes the jack can be removed and replaced. Take the music source to a repair shop.

Cut down the old jack.

Remove the cup from the new jack and pull up to the on the cable.
Clean up the wire about 1 cm long. There should be 1 common ground, one for Left and one for right channel.
Solder the Left channel's wire to the inner point.
Solder the Right channel to the middle point .
If you have two independent ground just spin together and solder them to the end point of the jack.
With plier press together the jack end, thi swill hold the wire
Pull down and twist the cap.

Test with another device such as an MP3 player or iPod. If it's still not working, replace the speaker.

Normally these low end system components depend on each other and cannot be interchanged with components from other system. So if one of the components (subwoofer or right control satellite speaker) breaks or gets lost the whole system is unusable...
I was in the same situation. Got a subwoofer from VS4121 without original satellite speakers. I acquired satellite speakers from Altec Lansing 251 which used same connectors but the internal wiring was different and incompatible. Speakers are also glued together so it is impossible to open without damaging it. Obviously there is no service documentation anywhere..
But if you like electronics you can try to make something based on the information below:

Altec Lansing VS4121 connector pinout:
Take the male connector (from right satellite speaker) and number pins in first row from left to right as 1-3, pins in the second row as 4-7, pins in the bottom row as 8-9 and the circle enclosure as 0. Then:
0 - Ground
1 - Speaker Right
2 - Treble (seems just like voltage control but may need to be confirmed: Min=0V; Max=voltage from Volume control Pin 9)
3 - Head Set Right
4 - Bass signal (not sure what the exact original design is as I have not opened the speaker) But seems like they use 50K potentiometer and some sort of RC low pass filter configuration. When potentiometer is set to Min it is shorted with ground. When set to Max it is taking signal from Right and Left Head Set (pins 3 and 5) combined through some larger resistors and low pass filter.
5 - Headset Left
6 - Ground for Headset (grounds 0 and 6 seems to be connected in the subwofer but within the right satellite speaker they seem to be isolated with separate leads to sub)
7 - Power ON/OFF. When OFF or headset jack plugged then 0V. When ON and headset jack unplugged then 5V
8 - 5V Power in
9 - Volume (min=0V; max=5V). It seems they use 50K potentiometer but probably anything >10k should work.

At minimum for standalone subwoofer you will need to implement:
- Power On/Off (switch between pins 8,7)
- Volume control 50k potentiometer (probably anything between 10k and 50k will do) between pin 7 and 0 with variable output to pin 9
- Bass control - join two resistors R (I used 7k) and call this join point A. Attach other ends of the resistors to pins 3 and 5 respectively. Attach 50k potentiometer (I used 10k) between point A and ground 0. Attach Variable pin of the potentiometer to pin 4 (Bass input signal). Attach capacitor C between Pin 4 and Ground pin 0 (I used Electrolytic 1uF with negative end to ground) . Resistor R and capacitor C should make simple passive low pass filter (Google for formula) that should short any frequency higher than 80-120Hz to ground and feed frequencies lower than 80-120Hz to pin 4.

PS: I managed to hack 251 control satellite speaker to somewhat work with VS4121 Subwoofer but that is another subject. All electronic parts were salvaged from old broken TV so total cost was $0 but took several days of tinkering for speakers that I could buy used for $10-20 :)

can anyone help me with the Altec Lansing VS4121 2.1 Channel i need the wiring diagram of 9 pin cable i broke it. please mail me the layout at [email protected]

The answer is NO... These jacks need to float driving speakers that are NOT grounded in any way. Connection to channel A with a grounded cable will destroy the channel A amp. Use the 1/4 inch MAIN L & R jacks with balanced lines to active speakers. You can drive several active speakers from each of these jacks.

I'm thinking you have an open shield ground on the RCA input side. Connecting speaker inputs possibly restores the ground. Try connecting the high level inputs then disconnect the remote end of the cables (floating the grounds).

Then get out an ohmmeter and find that open circuitor or bad solder joint between RCA ground and real ground in the speaker's amplifier. Or.... if speaker ground kills the hum and you want to use RCA Line Level input to the sub, just connect one minus speaker output on your source amp to one minus on the sub's amp.

The black wire goes in the external speaker and the red wire goes in the PA jack.

1ST.-The right jack could be damaged!!
Usually, there is a [+} posaitve & [-] negative input\output to speaker connections. Not all [-]neg. is ground.Sometimes the [+] pos. is grounded. [The exception to the rule is that u SEE the black wire going to ground] the reason I am letting u know this is that u can test the INSIDES of the sound system.
If u can get inside of the unit. Cut the left speaker output [+] posative from both outputs. criss\cross wires.Place left speaker output to right speaker jack[ that does not work!!] & visa versa.If the [LEFT Output] stops working after u plug in the speakers,the internal RIGHT output is damaged. NEEDS TO BE TESTED
If nothing nothing changes.The left still works & the right still does not work, then it is the jack (shorted ?, opened internally ?)