We have analyzed the differences in the oscillation eigenfrequencies measured in the last three years of GONG (From September 1996 to August 1998) observations and 2 years of SOI/MDI (from May 1996 to May 1998) observations. We have inverted these data to look for signatures of the solar activity cycle on the stratification. A one-dimensional (radial) structure inversion code was used to study temporal variations of the "bump" in the sound speed distribution located in the boundary between the radiative and convective zones. These variations could be an indication of a toroidal magnetic field anchored in this region. A two-dimensional (radial and latitudinal) structure inversion code was used to analyze surface latitudinal variations of the sound speed distribution with time. We also attempted to push the inversion to deeper regions, but we failed to produce well localized solutions due to the noise present in the data. The most interesting results are those found for the sound speed distribution in the solar surface. There is a clear dependence of the angular distribution of sound speed with time. The relation between this dependence and some physical phenomena like the rising of the magnetic activity or thermal variations in the solar surface are explored.