How is the voice quality of a seiyuu classified?
Well I don´t think they actually classify the quality of each voice. Why do I think so?

Most seiyuu are picked after they either send in some records or do a vocal audition, or they have done voice overs/acting before and are scouted this way. 

When a company wants some voice acting done, they usually keep in mind the characters that have to be voiced. Most of the time characters have a specific personality and they want a voice to best represent this personality. For example, let's look at Mizuhara Kaoru, the seiyuu who voiced Kusakabe Misao in Lucky Star. She has a melodic yet playful voice which can represent the character as one of the best. To even better grasp this concept, think of Naruto being voiced with Kusakabe Misao's voice. This would not match the personality the character has to represent.

So if there were to be a classification for seiyuu, it would differ per character. So there are no known "standard classifications" for seiyuu.