Audiobooks are an exclusive feature of NetGalley Advanced*. If you are a NetGalley Advanced client, press the Create New button on the MANAGE TITLES page. Much more detail is outlined in this article, and don't forget to Associate Other Formats you may have uploaded to NetGalley.
Once you have created your title record, you must upload your audiobook files to the FTP.
*Please contact concierge[at]netgalley.com if you would like to upgrade to NetGalley Advanced.