I don't own an Audax, but for all my bikes including some randonneur bikes that are very similar to the Audax, I prefer my Ortlieb Ultimate V handlebar bag in size Large with 8.5l capacity and two small zippered mesh side pockets. The Large weighs about an aspirin tablet more than the Medium but holds a lot more, nice for light but bulky things like wind jackets and such.
I've found its greatest virtue is reliable waterproofness. It has withstood more rainstorms than I can count, all without a leak.
I prefer the older Ultimate V because it uses very secure snaps to close the lid and secure the map case instead of magnets, which are located in a way that would be problematic for my portable hard drive and credit cards carried inside. Otherwise, the newer Ultimate VI is really nice also.
On my bikes with threadless steerers, I place the bag mount low on a 55mm Thorn Accessory T-bar instead of directly to the handlebars. This puts the weight a smidge lower for better handling.
Best,
Dan.