If you need to be able to change the tone "continously" then rearanging the code slighty might make it a little bit better, the glitch should be in the us* range:
Code:
Main: 
   GO_DONE = 1		    ' Start the conversion
   IF Trigger THEN       
      Sound SPK,[Tone / 4 + 50, 1] 
   ENDIF
   WHILE GO_DONE : WEND	' If trigger isn't 'on' we need to wait for the ADC.
   Tone = ADRES
Goto Main
*Depending on how much time SOUND needs to "set it self up".

/Henrik.