INT 66 - IBMSND driver, DIGPAK - SET CALLBACK ADDRESS                           
	AX = 068Eh
	BX:DX -> callback function
	    0000h:0000h to disable callback
	DS = value to load into DS when calling the callback function
Desc:	specify the function to be called when playback of a sound effect is
	  completed
Note:	the callback function will typically be called during a hardware
	  interrupt, so all the usual precautions should be taken except for
	  preserving registers
SeeAlso: AX=0691h