There has been speculation over the 4/29 Sprint event named "Hear It For Yourself" that it will be the official launch of HD Voice service on the network. There may be other topics discussed such as WiFi calling and VoLTE roadmap discussion etc.
Recode is reporting that Spotify and Sprint will announce a partnership supporting carrier billing and possibly discounts for premium Spotify service.
http://recode.net/2014/04/11/sprint-and-spotify-plan-to-partner-up/