The influence of the wall vibrations of a musical wind instrument on tone quality remains an open question. In order to quantify the effects of these vibrations, a model of the vibroacoustic behaviour of a simplified instrument (clarinet-like instrument) is proposed. The reed, which is represented by mechanical and acoustical harmonic sources, excites a thin cylindrical shell, filled and surrounded with air. The sound radiation due to wall vibrations has two origins, which are decoupled in the model making use of artificial baffles. The first one corresponds to the direct radiation of the shell in the external fluid. The second one is created by the internal radiation of the shell, which is then radiated outside the tube, through its open end. Three kinds of vibroacoustic couplings are involved in this situation: structure/internal fluid, structure/external fluid and inter-modal acoustic coupling due to sound radiation at the open end of the duct. A modal formulation of the problem is proposed which takes into account these three couplings. Impedances describing the shifts of the internal acoustic resonance frequencies due to the effects of the three kinds of couplings are given and permit one to quantify the wall vibrations effect.