A sensitive tuning of dielectric properties under the application of magnetic field in Fe74.4Co21.6Si0.5B3.3Mn0.1C0.1/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 multiferroic composites is presented. We demonstrate large variations of the effective permittivity at resonance region under extremely low magnetic field of 4 Oe near room temperature. The tuning of the maximum dielectric permittivity at resonance and the corresponding frequency was systematically studied. We suggest that the magnetic dependence of the effective permittivity is consistent with a strain-mediated magnetic-elasto-dielectric effect. Our results suggest a route towards future applications of the dielectric properties tuning effect in advanced devices.