A eo, A ea, Ae ie. Ooo! I ee a ooaa
On the island of Protocosmo, which floats in the middle of the earth, the inhabitants speak a musical language made up of six vowels: A, E, I, O, U, and OO. Their alphabet has no consonants, as such sounds are deemed offensive. (This island was described by Giacomo de Seingalt in 1788).
An alphabet without consonants seems very limiting, but English has quite a few all-vowel words. In fact, you can say such things as "I see a three-toed sloth" and "Good grief! A violent whirlwind!" with words made only of vowels. Here is an all-vowel poem: A eo, A ea, Ae ie. Ooo! I ee a ooaa.
Luckily, there's the Dictionary of All-Vowel Words.