I suspect that the file path isn't quite right for this. But keep in mind that I'm just guessing here.
Set Variable [$path; Value:"file:/Macintosh HD/Users/jdhelle/Music/Edison Cylinders/"]
I think you need the filepath format typical of an externa data source reference instead of what you find in the container field after the insert.
I tried "file", but it still brings up the dialog.
According to the help file, you'll get the dialog if there is no specified file name and path. I suspect that this will also be true if you don't set up your file path and file name to be exactly correct.
I found the issue. When I used 'Insert File' if the MP3 did not exist it just continued. With 'Insert Audio/Video' that's not the case.