What's in a name?
The Japanese word for "Sora" has a few meanings in translation. Officially, Sora's name is presented in katakana, perhaps to create a sense of otherworldliness. Japanese media, games, and anime write character's name in katakana often in order to denote that the word is foreign or fantastical. However, "sora" is lends itself from Japanese word and can be written in different ways.
The most common way of writing "sora" in Japanese. This particular kanji, as a noun, it means either sky or the heavens, and is read as sora. The meaning of sky is quite emphasized as the closest to what Sora's actual name might be. This corresponds to Kairi's and Riku's name, which mean sea and land, respectively. This coincidence was intentionally created in order to symbolize the connection between the three friends.
However, another meaning to the kanji is kara, which means empty (the most famous use of these reading is the word karate, a form of martial arts that translates as "empty hand"). This is quite ironic, in that Sora has, on two separate occasions, become "empty". His first would be the time he became a heartless; he had unlocked his heart, losing it to the darkness, and essentially becoming an empty shell. The second occasion is when Sora produces his Nobody, Roxas, who (supposedly) has no heart.
As an interjection, it can mean "Hey!" or "Look!" when written in hiragana. Although this reading isn't quite relevant to the character, it is an intersting note to point it.
Tetsuya Nomura has a knack for having the name of his characters nature themed. His male protagonists usually have names related to the sky, weather, or heavenly bodies. For example, Squall (FFVIII) is a violent storm. Tidus(FFX), used in Okinawan, means "sun", and of course, Cloud(FFVII).