To correct this, open the MIDI volume control and turn off Mute. 

In Windows 3.1, open your sound card's mixer and check the volume level for 
MIDI (or Synth) and verify that you do not have a Mute button selected. 

In Windows 95, open Start | Programs | Accessories | Multimedia.  In this 
group, open the program Volume Control.  There is a volume slider labeled 
"Synthesizer" with a box under it labeled "Select."  Place a check in this 
box to turn the mute off.

If the problem persists, you will need to determine if the problem is 
specific to the Discovering Music software or to the sound card playing 
MIDI (.MID) files (which the Jammin' Keys keyboard utilizes).  

To test this, open Media Player from the Multimedia group using the 
instructions above.  Select Device, and then MIDI Sequencer, to open a .MID 
file.  If you do not hear sound with this program either, contact your sound 
card or MIDI interface manufacturer to determine why the sound card cannot 
play .MID files.