This answer is from the Mad Scienctist website
The quick answer is that either type can be more memorable depending on the circumstances, but visual information is more easily remembered overall.
Part of what makes the answer complex is that what we see often causes us to think, and that thinking is remembered as if it were heard; so information presented visually might actually get stored twice: once in a visual "code" and once in an auditory code. There are other factors such as how motivated we are to remember that also can have a big effect.

Here's a more detailed answer, courtesy of my colleague Dr. Roger Remington:

Our capacity to store visual patterns for later recognition is remarkable,so if I had to choose between the two alternatives I would say the simpleanswer is that visual information is remembered better. The morecomplicated answer involves what precisely is meant by "remembered better"and whether this holds for all conditions. For lists of words, for example,I don't think there is much difference between visual and auditory modes ofpresentation. Even this is complicated because for auditory stimulus presentations visual imagery can help memory; likewise, for visual stimulus presentations auditory memory can help, so it's not clear we get a very pure measure of each.

It also matters whether people are (1) asked to recall what was presented or (2) simply to recognize a previously presented item. For recognition, people have an amazing ability to store complex visual information and to recognize and discriminate old from new pictures at retention intervals of days. However,people can also remember and hum tunes for years while they are only able to depict small portions of a scene they have been presented.
Part of the problem in answering this question is the nature of auditory and visual stimuli.

To oversimpify, a natural visual scene is interconnected spatially whereas an auditory stimulus in interconnected serially. This leads to differences in our ability to reproduce (recall) the event and in the amount of information needed to recognize a visual or auditory event among distracting events. If one chooses to look at the amount of information stored, then it would be the case that our visual information would win because of the rich representation of the world our visual system gives us.
This reasoning underlies the value of using spatial mnemonics (memory aids)to remember speeches and the like.It is important to keep in mind that in daily life what we remember is (toa first approximation) what we attend to. The modality (visual vs auditory)plays a role, but our motivation plays perhaps a more significant key role.In fact, the source of information is often lost with time: we remember that George Washington was the first president but are unlikely to be able to recall whether we heard that (audition) or read it somewhere (vision). Our memory for facts (semantic memory) can be separated from our memory for events (episodic memory). It is in episodic memory that we have the clearest recollection of whether we heard or read something. Visual and auditory memories can also reinforce each other. It is helpful to have both pictures and spoken words to support memory since that gives us a richer internal representation. A very powerful effect that works in both modalities is organization. If the material to be remembered is organized into a narrative or other structure it is remembered much better than if it is a disorganized hodge-podge.A book called "Human Memory" by Alan Baddeley is a good reference for this area though tough going for younger students.