[Icom] Vox operation on 756ProII

[Ray, W2EC]

I want to use software like MixW and use the VOX on the IC-756ProII for
PTT control. My soundcard interface is to the ACC1 port in the
IC-756ProII for both audio in and audio out. Everything works fine if I
manually activate transmit/receive via the front panel "TRANSMIT"
switch. But the VOX does not activate if I just initiate sending audio
via the ACC1 port. Vox works OK via the mic, but apparently not thru the
audio in on the ACC1 port. Is this correct or have I not gotten
something turned on correctly on the 756 menus to allow VOX to function
via audio in on ACC1?

Thanks, Ray W2EC