  Particle-like excitations, or quasi-particles, emerging from interacting
fermionic and bosonic quantum fields underlie many intriguing quantum phenomena
in high energy and condensed matter systems. Computation of the properties of
these excitations is frequently intractable in the strong interaction regime.
Quantum degenerate Bose-Fermi mixtures offer promising prospects to elucidate
the physics of such quasi-particles. In this work, we investigate phonon
propagation in an atomic Bose-Einstein condensate immersed in a degenerate
Fermi gas with interspecies scattering length $a_\text{BF}$ tuned by a Feshbach
resonance. We observe sound mode softening with moderate attractive
interactions. For even greater attraction, surprisingly, stable sound
propagation re-emerges and persists across the resonance. The stability of
phonons with resonant interactions opens up opportunities to investigate novel
Bose-Fermi liquids and fermionic pairing in the strong interaction regime.
