Due to the Pauli blocking of the intermediate states, the scattering matrix (or T matrix) of two fermionic atoms in a Fermi gas becomes different from that of two atoms in free space. We calculate  the in-medium T matrix in ladder approximation and study its effects on the properties of the collective modes of a trapped gas in the normal-fluid phase. We introduce the in-medium interaction on both sides of the Boltzmann equation, namely in the calculation of the mean field and in the calculation of the collision rate. This allows us to explain the observed upward shift of the frequency of the quadrupole mode in the collisionless regime. By including the mean field, we also improve considerably the agreement with the measured temperature dependence of frequency and damping rate of the scissors mode, whereas the use of the in-medium cross section deteriorates the description, in agreement with previous work.