You will receive this message if you are not using a full-duplex sound card 
and attempt to record multiple digital audio tracks.  

For further details, please see the section titled "I cannot record a 
digital audio track while another track is playing."

This message also occurs if the sound card is not capable of simultaneous 
MIDI and WAVE record/playback.  To prevent this error message from 
appearing, open the file "dop.ini" (located in the Windows directory) 
with Notepad, and add the line

  MIDIISWAVE=1 

to the [Options] section.  

Please note this only eliminates the error message from being displayed, it 
does not change the functionality of the sound cardhardware regarding MIDI 
and WAV playback.