Try to see if Speech works from your Speech options in the Control Panel:
Start Menu / Control Panel / Speech / Text to Speech tab / Preview Voice button
This may look different in your version of Windows.
I looked up "vlax-invoke" in AutoCAD help and found the following interesting info.
I'm wondering if "vlax-invoke" should really be changed to "vlax-invoke-method"?
I haven't tested it yet, but I thought it was interesting.
Code:
(vlax-invoke-method obj method arg [arg...])
This function was known as vlax-invoke prior to AutoCAD 2000.