As the femtosecond pulse propagates through a nonlinear medium, the nonlinearity generates new spatial and temporal frequency components. The phase of the complex pulse envelope explicitly depends on both spatial and temporal coordinates. The instantaneous frequency, proportional to the first time derivative of the phase, changes along the pulse. The input power of the pulse is high enough so that the pulse splitting inside the medium takes place. The pitch of the sound you hear is proportional to the instantaneous frequency.
Higher pitch corresponds to a blue frequency shift.