[Dx4win] PSK-31 Help!!!!

[Art - W6KY]

I have been running PSK-31 for a year simply using 2
cables from the CPU to the rig.  Line in and Spkr out
to mike plug of the 756PRO.  Suddenly my RX is coming
out of my PC speaker output triggering the VOX on the
756. So I am in a catch 22. The RX data goes right
back to the PRO instantly triggering TX on the PRO.  I
have never had this problem before and now I have it
on 2 computers.  This isn't a function of DX4WIN cuz
'any' line in signal comes out thru the speakers. I
have tried every setting with no success.  I can mute
the PC speakers, but then I have NO output PSK
signal..... HELP
73, Art  W6KY