I don't see the need for an interrupt for what you're doing there.
Code:
X = 5
Start:
   HPWM 3, X, 20000
   SOUND PortC.3, [255, 5]   ' Make sound for ~60ms
   X = X + 5
Goto Start
Though I'd probably setup the CCP module manually and write to the dutycycle register instead of using the HPWM command since I Believe you'll get some discontinuties in the output when constantly changing the dutycycle using HPWM - may be wrong on that and/or it may not matter in your case, try it.

/Henrik.