The zero-phonon line (ZPL) and the sideband in the vibronic spectrum of a single glyoxal molecule inside a (3)He droplet are analyzed within the framework of the Lax formalism. The new theory takes full account of the coupling of the molecule to the single particle-hole (PH) and collective excitations of the doped Fermionic droplet. The effect on the coupling of the wavevector dependence of the effective (3)He mass and the large local density of the first (3)He shell, resulting from the interaction with the chromophore, are also included in the theory. By fitting of a coupling parameter and the phase factor between the PH and collective response functions, the shape and relative intensity of the observed ZPL and its slowly decreasing multiexcitation sideband are well-reproduced. The new theory is consistent with the previous explanation of the surprisingly sharp phonon line superimposed on the sideband in terms of the dense first (3)He shell, which acts as a Helmholtz resonator for the zero sound of the droplet.