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We present a Raman scattering study of as-grown and annealed GaAsBi, GaNAs, and GaNAsBi epilayers grown by plasma-assisted molecular beam epitaxy. In particular, photo-excited longitudinal optical phonon-plasmon-coupled (LOPC) modes were investigated using excitation power-dependent measurements. The optical properties of the samples were characterized by photoluminescence and photoreflectance measurements. From Raman scattering, local vibrational modes and disordered activated modes originating from N- and Bi-alloying were confirmed. The strong photo-excited LOPC modes observed in GaAsBi can be attributed to hole trapping caused by the Bi-induced shallow localized states. The weak photo-excited LOPC modes observed in the as-grown GaNAs and GaNAsBi indicate rapid carrier trapping owing to the high defect density in these dilute nitride alloys. After thermal annealing of GaNAs and GaNAsBi, the heavily damped photo-excited LOPC modes were observed, which can be explained by the low electron mobilities related to the N-induced shallow localized states.