  We study low energy collective modes and transport properties of the "helical
metal" on the surface of a topological insulator. At low energies, electrical
transport and spin dynamics at the surface are exactly related by an operator
identity equating the electric current to the in-plane components of the spin
degrees of freedom. From this relation it follows that an undamped spin wave
always accompanies the sound mode in the helical metal -- thus it is possible
to `hear' the sound of spins. In the presence of long range Coulomb
interactions, the surface plasmon mode is also coupled to the spin wave, giving
rise to a hybridized "spin-plasmon" mode. We make quantitative predictions for
the spin-plasmon in ${\rm Bi}_2{\rm Se}_3$, and discuss its detection in a
spin-grating experiment.
