On my iPhone 6, the Sonos app shows a list of the alphabet letters down the right-hand side of the screen when I display the "songs" listing. If I slide my finger up & down the alphabet, a larger version of each letter appears near the lower centre of the main screen. I'd expect the main list to jump to that letter when I lift off my finger but nothing seems to happen at all. I would also expect that if I tapped on a single letter that the main list would jump to that letter, but again nothing happens (other than that the adjacent song sometimes gets sleeted because of fat finger syndrome). I'm attaching a screenshot here.

My songs are on a NAS drive (a Brennan B2) and I haven't yet grouped them into artists, albums etc as that info apparently doesn't get transferred from the B2. I have no trouble using this sort of alphabetic slider in (f'rinstance) the Contacts app on the iPhone but cannot see how it's supposed to work in the Sonos app.