A model based on Galbrun's equation is proposed to address the problem of vibro-acoustic interactions in sheared flows. The use of a displacement-pressure mixed formulation of Galbrun's equation greatly simplifies the coupling condition formulations and avoid the problem of non-zero frequency spurious modes encountered with displacement based acoustic formulations. This model is applied to duct acoustics. Comparisons with analytical models demonstrate the accuracy of the method. The effects of mean flow shear on acoustic wave propagation in elastic duct are then illustrated.