Singing rests on a physiology quite separate from a good speaking voice, which is why fine orators sing badly and fine singers are often unremarkable in conversation. The skill therefore carries its own primary quality, Voice, averaged with Creativity to give its Skill Base — the instrument and the musicianship, neither of which is worth much without the other.
A Singing Success Value test measures the aesthetic worth of a performance, read exactly as Dancing is: its Value Diamonds bear on persuasion for as long as the singing lasts, and a genuinely fine voice in a hall is a political instrument as much as an artistic one.
Where a song is performed with an instrument, the two are tested separately; a lutenist who cannot sing is well advised to hire someone who can.