OK, so someone with a really strong sense of their bearings and direction and good at finding their way, a route sheet should be easily doable?
At the minute I look up my routes on Google maps and then cast them to memory before going out, granted it's not 200 or 300ks worth of directions but around 50-70 miles.
You generally need to keep a good eye on your route sheet so you don't miss any of the controls or info controls. I use a GPS but also laminate my route sheet in small pieces and bulldog clip it to my bars where I can see it. If my GPS went pear shaped I could (probably) still complete the ride even with no map.
Personally the aspects of Audax I enjoy most are the riding, the scenery and chatting to other cycling nutters. I don't really enjoy navigation, and I enjoy a ride much less if I am constantly wondering if I am going the right way, a GPS basically gives me reassurance that I am on route. Oh, and I have no sense of direction and my knowledge of geography is crap too.
The best route sheets give a distance in km for each instruction, e.g. 'Turn L at T 56.2 km'
It is much easier to miss turns or even worse completely forget how far down the sheet you are if there are no distances on it to cross reference.
+1 to what Jaded said about forgetting 15 seconds after you just read the sheet what the next instruction is