Need to describe a singing voice

Maybe a 'bell-like' quality?

ethereal? Angelic, melodic, crystalline? Do you simply want a word to describe it or a metaphor?

Female voices are typically classified as; soprano, mezzo-soprano, and contralto. If your MC isn't familiar with these terms, this could be a starting point for synonymous references and/or metaphors (a lilting, weightless tone; a floating wave of bright joy; a reserved undertone of ominous portent; etc...) that would comport with your MC's vocabulary.

en.wikipedia.org/wiki/Singing

What's wrong with spring water as a description? Makes sense to me.

I'd probably go with words like pure, natural or sweet. I think ethereal or angelic also work well. I'd personally advise against melodic and crystalline because they're slightly funny words to describe a singing voice.

I wouldn't bother with words like soprano/contralto etc because all they really tell you is how high or low someone can sing (e.g. a soprano can sing higher than an alto). It looks like you want to describe the sound of the voice - rather than how high or low it is - so descriptive words will work fine.

In classical music, a voice is generally described as belonging to a certain Fach.

Wikipedia has a good article on it. Maybe reading through that will help you?
http://en.wikipedia.org/wiki/Fach

I think you are going for a lyrical soprano.