George Karam has already given the precise explanation, the reason why two pronunciation exits is that relates to how Japanese make "いく" sound. When the people speak "いく" vaguely or ambiguously, though most Japanese are insensible, you hear "ゆく" alike. For that reason, the singers out loud "ゆく" clearly instead of "いく" sound.

From what I know there is no semantic difference between いく and ゆく, but いく tends to be used more for regular speech and is the more formal of the two, while ゆく is very prevalent in music and poetry and is less conversational. I believe ゆく is used most songs to get the rhyme just right or keep rhythm. But other than that they mean the same thing.