It is worth remembering that 'quality' is also used to describe traits, without the level of judgment implied by the two definitions above.

Dictionary.com Unabridged (v 1.1)
qual·i·ty      /ˈkwɒlɪti/ [kwol-i-tee] noun, plural -ties, adjective
–noun
1.        an essential or distinctive characteristic, property, or attribute: the chemical qualities of alcohol.
2.        character or nature, as belonging to or distinguishing a thing: the quality of a sound.

I suspect this aspect of 'quality' was what was meant in referring to Ms. Bunyan's and Ms. Collins' voices as possessing an unusual other-worldly quality.