Hello,
Downloads can not be reliably recorded via auditing. E.g. if a user clicks the download action, but then "Abort" in the browser Download dialog, you still get an event on ContentService.getReader (which you might use to audit downloads), when the browser checks the size and mimetype of the file about to be downloaded.
I gave up on (reliable) download auditing - especially if you try to achieve this with respect to the different protocols provided by Alfresco (i.e. what about a SharePoint protocol "download", which is required prior to editing a file?).
Regards
Axel