Written Sound is a site which looks at onomatopoeia (words that imitate sound). Here are the entries for the letter A.
It's interesting to compare the words used to represent sounds in different languages. In French, for instance, the word used for the sound of sneezing is 'atchoum' (very similar to English 'atchoo') while a French cow goes 'meuh' as opposed to an English one, which goes 'moo'!
