In this study, coherent phonon motion is observed in bandgap excited CuIn${}_x$Ga${}_{(1-x)}$Se${}_2$ (CIGS) extreme-ultraviolet (XUV) attosecond transient absorption spectroscopy across the Se M${}_{4,5}$ absorption edge. Two frequencies of coherent phonon motion are resolved, a low frequency mode attributed through Raman measurements to the $A_{1g}$ phonon motion of a Cu-deficient ordered vacancy compound (OVC), while the high frequency mode originates from the $A_{1g}$ phonon motion in the chalcopyrite phase. The two oscillations lead to modulations in the XUV differential absorption $\Delta A(ϵ,\tau )$ due to energy shifts of the Se M${}_{4,5}$ edge, with a minimum of the coherent-phonon amplitude occurring approximately 1 ps after band-gap excitation. The picosecond recoveries of the valence-band state-opening and conduction-band state-blocking signals are separately retrieved. The valence- and conduction-band signals recover with time constants of 1.4(1) ps and 2.2(1) ps, respectively. The conduction-band value agrees well with the approximately 3-ps carrier-cooling time previously reported for comparable co-evaporated CIGS. These constants characterize the recovery of the XUV state-blocking amplitudes and are not direct measurements of the total carrier densities or unique microscopic cooling times. Most importantly, we also observe fast oscillations (18.6(3)-fs period) across the Se absorption edge, which are interpreted to originate from quantum path interference between the electronic conduction bands of the chalcopyrite CIGS and OVC phases, providing a femtosecond probe of interfacial electronic coherence in a photovoltaic absorber. The complex interplay between the chalcopyrite and OVC phases is revealed in this investigation through both coherent vibrational and electronic motions.