You're wrong about one thing...
Sound/music for this beautiful animation will not be simple to add in & may take up as much time as the animation did. I still enjoyed it even with 0 sound because the graphics were so good (even though the frame rate was indeed a tiny bit choppy when he's riding his bike... & I have a FAST computer). One thing that was both good & bad was the overly-cartoony way in which he pressed the button. It was funny, but nobody would ever press a button that way. Still, I'm guessing you weren't really going for realism. ;)
I'd say that for music this animation needs a specially composed orchestral-sounding score to correspond with the action on the screen, something to really give it atmosphere in some places & more drama in others. You know the animation "Bitey of Brackenwood"? If you go to the website of the creator, and go to the message board, there's a big group of composers there in one section (some of whom are very good) who may be willing to help you.
For sound I imagine you know much better than I do what it needs... a realistic rain sound, of course, but some cartoony sounds as well.
I do think that adding sound/music in would be worth it... for one thing, I think that if it had music/sound this flash would be rated in the top 10 right now. Adding sound really does make people like something a lot more (if the sound is good).
Great job, & good luck in your studies!