Unable to create File download Product from an Audio node

Drupal 5.4
Audio 5.x-1.3

I created an Audio node. The node displays and plays fine.
I edited the Audio node, selected "Product > File download product", and clicked the button labeled "Add to store".
The Edit node page displays again, this time with File Download Product fields. "File path:" is blank and NOT editable. Below the field it says "Do not edit. File is managed by audio node.". I click the "Submit" button at the bottom of the page, and the form submission fails validation with the message "Please enter the path to the file."

Troubleshooting
I was able to successfully turn an Audio node into a "Nonshippable Product", and the presentation of this product is perfect! So I know this feature COULD work.

Please help when you get time. Thanks

-Stephen