I've had this theory for a while and I hope I can properly articulate it here so I get people's opinion on it. I got his idea mainly from two places:
1) At the end of book 2 the listeners, after becoming what I assume are Regals and attaining a form of power related to lightning, start singing in unison to summon the Everstorm.
2) Rlain and several others mention something they name "the song of Roshar" and that they, as Parsh, attune themselves to some aspect of it. Rlains comments lead us to think that humans can feel it too.
These two things together led me to believe that Singers, when using any form, could unite together could sing to manipulate Roshar and apply whatever their form is associated with to a much larger scale. This could potentially be how the Dawncities were created and how Shinovar was terraformed, a huge group of singers using either one form of power or a mixture of them got together and sang.
I find the whole concept of the song of Roshar or hearing Roshar as Rlain says very interesing and I hope get more tidbits about this in the next book focused on Venli.