  We make use of micro-magneto Raman scattering spectroscopy to probe
magneto-phonon resonances (MPR) in suspended mono- to penta-layer graphene. MPR
correspond to avoided crossings between zone-center optical phonons (G-mode)
and optically-active inter Landau level (LL) transitions and provide a tool to
perform LL spectroscopy at a fixed energy ($\approx 197~\rm{meV}$) set by the
G-mode phonon. Using a single-particle effective bilayer model, we readily
extract the velocity parameter associated with each MPR. A single velocity
parameter slightly above the bulk graphite value suffices to fit all MPR for
$N\geq2$ layer systems. In contrast, in monolayer graphene, we find that the
velocity parameter increases significantly from $(1.23\pm 0.01) \times
10^6~\mathrm{m.s^{-1}}$ up to $(1.45\pm0.02) \times 10^6~\mathrm{m.s^{-1}}$ as
the first to third optically-active inter LL transition couple to the G-mode
phonon. This result is understood as a signature of enhanced many-body effects
in unscreened graphene.
