  We demonstrate that single-layer graphene in the presence of a metal gate
displays a gapless collective (plasmon) mode that has a linear dispersion at
long wavelengths. We calculate exactly the acoustic-plasmon group velocity at
the level of the random phase approximation and carry out microscopic
calculations of the one-body spectral function of such system. Despite
screening exerted by the metal, we find that graphene's quasiparticle spectrum
displays a very rich structure characterized by composite hole-acoustic plasmon
satellite bands (that we term for brevity "soundarons"), which can be observed
by e.g. angle-resolved photoemission spectroscopy.
