Full honest review time.
Story: I found that the plot you outlined was touching and almost a little cruel. It's beyond my range of comprehension to truly feel sympathetic with the main character (no tears shed today), but I feel almost horrified after watching that. Unfortunately, I almost feel my horror is unjust, since this scenario has a possibility of actually occurring. That may be why I feel this way. As opposed to looking at the zombie apocalypse scenario as a gun and violence-filled wonderland, truly appreciating the possible psychological horrors which may occur while one strives for survival is tough.
Graphics: This is probably the most difficult for me to comment on, since I generally prefer single-toned animations. Multiple types of animations in the same picture get to me after a little while. Regardless, I feel that the use of two different scenes like that really accentuate the girl's madness. However, in addition to the scenery, one of my favorite parts about the graphics is the font you chose. Such a violent and erratic typeset makes you consider how mentally frail she really is in such a state.
Audio: Yes, yes, you stated that the audio quality suffered a bit at the end. I only wish the exact opposite were true. While her writing symbolized her mental strength (which is why it was strongest when "with" friends at the end), the music stayed melodic and almost harmonious throughout, albeit a little bit creepy, especially by the end.
Overview: The audio was the area which put this movie at a 4.5 stars for me. The story was fantastic, the graphics were artistic on multiple levels, and the audio made sense and matched the tone. Simply, at the end, the repetition started annoying me. With this in mind, such mindless repetition only accentuates the girl's loss of mental comprehension. Great film. I hope you can make more of these types (ones where thinking more about them lets you appreciate them more).