  By solving the equations of ordinary and two-fluid hydrodynamics, we study
the oscillatory modes in isotropic nonpolar dielectrics He I and He II in the
presence of an alternating electric field
$\textbf{E}=E_{0}\textbf{i}_{z}\sin{(k_{0}z-\omega_{0} t)}$. The electric field
and oscillations of the density become ``coupled,'' since the density gradient
causes a spontaneous polarization $\textbf{P}_{s}$, and the electric force
contains the term $(\textbf{P}_{s}\nabla)\textbf{E}$. The analysis shows that
the field $\textbf{E}$ changes the velocities of first and second sounds,
propagating along $\textbf{E}$, by the formula $u_{j}\approx c_{j}+\chi_{j}
E_{0}^{2}$ (where $j=1, 2$; $c_{j}$ is the velocity of the $j$-th sound for
$E_{0}=0$, and $\chi_{j}$ is a constant). We have found that the field
$\textbf{E}$ jointly with a wave of the first (second) sound $(\omega,k)$
should create in He II hybrid acousto-electric (thermo-electric) density waves
$(\omega + l \omega_{0},k + lk_{0})$, where $l=\pm 1, \pm 2, \ldots$. The
amplitudes of acousto-electric waves and the quantity $|u_{1}-c_{1}|$ are
negligibly small, but they should increase in the resonance way at definite
$\omega$ and $\omega_{0}$. Apparently, the first resonance corresponds to the
decay of a photon into two phonons with the transfer of a momentum to the whole
liquid. Therefore, the spectrum of an electromagnetic signal should contain a
narrow absorption line like that in the M\"{o}ssbauer effect.
