Influence of optic wavelength dispersion on phase matching conditions of acousto-optic interaction in optically biaxial media is considered for the first time. Diffraction of light by ultrasound is examined in basic optical plane of a biaxial crystal by the example of monoclinic double lead molybdate single crystal (Pb₂MoO₅). Frequency dependences of Bragg angle have been analysed taking into account wavelength dispersion of light. It is shown that a considerable transformation of the characteristics of the acousto-optic interaction is caused by a change of the directions of optic and dielectric axes in a biaxial medium with a tuning of optic wavelength.