  We study the interplay between collective electronic and lattice modes in
polar metals in an applied magnetic field aligned with the polar axis. Static
spin-orbit coupling leads to the appearance of a particle-hole spin-flip
continuum that is gapped at low energies in a finite field. We find that a weak
spin-orbit assisted coupling between electrons and polar phonons induces the
emergence of electronic collective modes. The strength of the applied magnetic
field tunes the number of modes and their energies, which can lie both above
and below the particle-hole continuum. For a range of field values, we identify
Fano-like interference between the electronic continuum and phonons. We show
that signatures of these collective modes can be observed in electron spin
resonance experiments, and we provide the corresponding theoretical
predictions.
