Abstract: Mode coupling plays a dominant role in the fluctuations of sound signals at the receiver for sufficiently large inclination angle of the internal soliton (IS) front relative to the acoustic path. The coupled-mode formulas for sound filed fluctuations are given in this situation. Theoretical analysis is carried out for intensity fluctuations due to the motion of ISs in shallow water, and numerical simulations are also performed using a parabolic equation model. The quasi-periodic variations of intensity can be observed from the numerical results. The peaks in the spectrum of intensity take place at the so called predominant frequency and its higher harmonics. The predominant frequency is proportional to the velocity of the IS motion along the acoustic path and inversely proportional to the scale of interference beating of the interacting modes, which corresponds to the cycle length of the critical ray in the ray theory. The predominant frequency value does not depend on the IS shape. Moreover, the vertical structures of the intensity spectrum are analyzed and shown to correspond the shapes of the modes which contribute significantly to intensity fluctuations.