The behaviour of the sound velocity and damping in the phase transition vicinity in the weak ferroelectric Li₂Ge₇O₁₅ has been investigated by the ultrasonic technique (30-150 MHz) and by Brillouin scattering (about 40 GHz). A large critical contribution to the acoustic anomalies has been found in the paraelectric phase. Analysis of the sound velocity and damping dispersion revealed the relaxation parts of anomalies under conditions when the critical parts are large. The Landau theory is used to interpret the results obtained. Some coefficients in the thermodynamic potential expansion are determined. Increasing suppression of the relaxation anomalies by the depolarizing field as T to T_c is considered for both the sound velocity and the damping of longitudinal phonons with the wavevector along the polar axis. This is a specific feature of weak ferroelectrics.