How to Rewire IBM MM55/75 Monitor
Audio Cable to Fit a Standard Sound Card

* Read below statement first

Option 1

Step 1 - Cut the audio wire within an inch of the purple plug.  This
will leave you plenty of wire to work with.

Step 2 - Strip about two inches of plastic and shielding off the
audio wire.  Six smaller colored wired wires will be exposed.

Step 3 - Strip about one inch of plastic off the red, black and
beige wires.

Step 4 - Connect two positive stereo speaker wires; one to the red
monitor wire and one to the black monitor wire.  Then connect both of
the negative speaker wires to the beige monitor wire.  Then plug the
speaker wires into an amplifier.

Step 5 - Connect to the computer a mini plug that leads to two RCA
plugs.  Connect the RCA plugs to the input of the amplifier.

Now the only obstacle to being completely back to normal, is that I have to run 
the computer sound through a stereo amplifier.  This is because the original Aptiva systems these
monitors were designed to work with contain a Bose Amplifier in which the audio cable was made to fit.

Option 2
by "cormac" (aka Dave)

You can buy a 7-pin mini din female connector at an electronics supply
store... (see note below) and get a small size stereo jack.... for about 3-5 dollars cnd.

Looking at the mini din (see image below) you will see three pins in a row at the keyed edge
of the connector.... the center should be connected to the ground lug of the
stereo jack.... the left pin to the center lug of the jack.... and the right
pin to the tip lug of the stereo jack.... ignore the rest of the pins in the
mini din.

The stereo jack can now be plugged into the (out) of the sound card and the
monitor's plug into the new plug........ note the volume control nor the mute button on the monitor will work... 
but you can adjust the sound using windows.
Also note if you get the left and right pins backwards the adapter will
still work...but if you adjust your sounds' balance the left and right
speaker will be reversed.  (not really a problem, just aesthetics)....
I've done this and it works!

As of 7/22/1999

For those interested Digi-Key (http://www.digikey.com/)  will be getting a new supply of the 7 Pin female
      Mini-Din Connectors soon.... On back-order for 12-14 weeks. This connector adapts to the one
      coming from the monitor to the bose audio amp input. You can use this connector to adapt to
      a new sound card connection along with a stereo plug connection.
      Digi-Key part number is CP-2170-ND. Price $1.32

Thanks goes to Dave J. for providing Digi-Key and part info!

* These rewiring procedures are in no way supported or endorsed by IBM.  
Option 1 was posted on a public web site by another Aptiva owner who got an
MM55's speakers working on his 2159 Aptiva.  The more than helpful information
was passed on to him by a Radio Shack employee.