Hi,
This is caused by the MIDI-IN /MIDI-OUT loop you (probably) created with the MIDI-cables.

Your MS keyboard sends the key-information to :
1. the internal Sound-generator
2. the MIDI-out connector

So, without the midi-cable only your MS will sound.

With the midi connected
a. keyboard MIDI-out to MIDI-in on the SB
b. SB-MIDI-out to keyboard MIDI-in
you are able to let the SB-synth as well.

In many pragrams (e.g. Cubase, Logic) the user is able to "connect" the computer MIDI-in with the MIDI-out : MIDI-THRU set to ON.

I expect this is the case in your computer.

There are two ways to solve your 'problem' :

a. In your MS keyboard : Disconnect the internal connection keyboard --> soundgenerator. This is called the LOCAL-OFF parameter. Played notes will be send via the computer if MIDI-THRU is set to ON.

b. In your PC : Switch off the MIDI-THRU option in the sequencer software.

regards,

Roel