The sound-pressure response of an acoustic-structural coupled system to both air-borne and structure-borne excitations has been studied. Both steady-state and transient-state sound field responses of the system can be characterized by the system modal behaviors and by the nature of the driving forces. This paper presents a method to describe the sound and vibration fields of the coupled system in terms of modal coupling analysis. In particular, analytical expressions for steady-state and transient-state sound-pressure fields in a panel-cavity coupled system are obtained. They are used to calculate the frequency response and transient decay of the sound field in the cavity. The effect of the system eigenvalue distributions (or the pole locations in the sound field transfer functions) upon the sound field response is demonstrated by numerical examples.