I've had this problem with some NI synths, but never saw it on Rapture before. There's not automation (that I can see) but whenever I start playback the master volume jumps back to 20%. Ghosts?
 

I recall having that issue with Battery 3 a while back.  Check your volume on the midi track for Rapture.  I dimly remember the Battery 3 issue as its internal master volume was tied to the midi track volume - moving it in the track would impact the Battery 3's master output volume.
 
Or not - my brain is the functional equivalent of a still life at times.  :)
 
Bob Bone
 

Yup what Bob says, check post #17 of:
http://forum.cakewalk.com/Solved-NI-Battery-equivalent-in-SONAR-X3-natively-m2986110.aspx
 
... and try and perform the steps with Rapture I suggest.