Start FSX. From the Free Flight menu, select the C172 and click on Fly Now. Once your flight is ready, shut it down using CTRL+SHIFT+F1 to  turn the mixture off, turn everything else OFF EXCEPT the Avionics Switch. Save the flight and make it the default flight. Now when you start FSX and you start with the 767, you will have Avionics power already ON. Now you will be able hear the ATIS messages when you tune in, as well as communicating with the ATC.

Note: I have selected CTRL+SHIFT+A as a keyboard shortcut for turning the Avionics On in FSX.

Hope this helps.

Mark