  WMAP observations have accurately determined the position of the first two
peaks and dips in the CMB temperature power spectrum. These encode information
on the ratio of the distance to the last scattering surface to the sound
horizon at decoupling. However pre-recombination processes can contaminate this
distance information. In order to assess the amplitude of these effects we use
the WMAP data and evaluate the relative differences of the CMB peaks and dips
multipoles. We find that the position of the first peak is largely displaced
with the respect to the expected position of the sound horizon scale at
decoupling. In contrast the relative spacings of the higher extrema are
statistically consistent with those expected from perfect harmonic
oscillations. This provides evidence for a scale dependent phase shift of the
CMB oscillations which is caused by gravitational driving forces affecting the
propagation of sound waves before recombination. By accounting for these
effects we have performed a MCMC likelihood analysis to constrain in
combination with recent BAO data a constant dark energy equation w. For a flat
universe we find at 95% upper limit w<-1.10, and including the HST prior
w<-1.14, which are only marginally consistent with limits derived from the
supernova SNLS sample. Larger limits are obtained for non-flat cosmologies.
  From the full CMB likelihood analysis we also estimate the values of the
shift parameter R and the multipole l_a of the acoustic horizon at decoupling
for several cosmologies to test their dependence on model assumptions. Although
the analysis of the full CMB spectra should be always preferred, using the
position of the CMB peaks and dips provide a simple and consistent method for
combining CMB constraints with other datasets.
