There are two types of royalties for music played on streaming websites such as Pandora: performance royalties (for the performers of the music) and publishing royalties (for the writers/owners of the music).
 
Pandora pays statutory performance royalties to SoundExchange, as well as publishing royalties to ASCAP, SESAC, and BMI.

Many musicians are familiar with ASCAP, SESAC, and BMI, but unfamiliar with SoundExchange. To register with SoundExchange, please visit http://blog.pandora.com/info/contents/soundexchangeregistration.html

Artists and record labels can register with SoundExchange to collect their portion of the royalties paid by Pandora and other Internet Radio streamers.

Similarly, songwriters collect their publishing royalties from ASCAP, SESAC, or BMI. They are negotiated separately by all legal music services including Pandora.