If i sign out of the find my iphone app will the sound still go off? If I have pressed the "play a sound" button in the app will the sound still play even after I sign out of the app?
Short answer: Yes.

Long answer: The app is an interface for the FindMyiPhone service. This service is controlled in the iCloud menu of settings. So unless you disable Find My iPhone in the phone's settings, the sound will play.