An undervoice (orignally a term in choral voicings) is an accompanying note, not the melody (nor the bass line) So what it's saying is that the writer wants to hear the major 7th in the chord, even though it isn't the melody note. If it weren't played, the chord could be heard as a dominant chord. many times chords in a song are made more 'bluesy' by making everythign a dominant 7th chord. However, this sounds a little, dull, and so having major and minor 7th chords in a progression makes it sound richer or more refined. So I'd guess the writer wants to make sure you include a natural 7th in the accompanying chords you comp in that measure. In a Berklee book on guitar there isn't going to be any mention of sotto voce, I don't think. That's a classical choral thing- borrowed from its original usage as a theater term, I believe.

Hello Lugus. "Under voice" or "sotto voice" means to speak under one's breath very quietly. It's a dramatic lowering of the vocal or instrumental tone [ in your case guitar music] with a hushed quality. An example of sotto/undervoice . is in the Lacrimosa from Mozart's Requiem Mass in D Minor. The singers lower their volume in order to attain the 'hushed' quality desired. Good luck with your guitar playing!The Beatlles' song "While my guitar gently weeps" is an example of "undervoice".

I am happy to help you anytime Lugus.