Dynamic magnetoelectric coupling in the improper ferroelectric Cu1−xZnxO (x=0,x=0.05) was investigated using terahertz time-domain spectroscopy to probe electromagnon and magnon modes. Zinc substitution was found to reduce the antiferromagnetic ordering temperature and widen the multiferroic phase, under the dual influences of spin dilution and a reduction in unit-cell volume. The impact of Zn substitution on lattice dynamics was elucidated by Raman and Fourier-transform spectroscopy, and shell-model calculations. Pronounced softenings of the Au phonons, active along the direction of ferroelectric polarization, occur in the multiferroic state of Cu1−xZnxO, and indicate strong spin-phonon coupling. The commensurate antiferromagnetic phase also exhibits spin-phonon coupling, as evidenced by a Raman-active zone-folded acoustic phonon, and spin dilution reduces the spin-phonon coupling coefficient. While the phonon and magnon modes broaden and shift as a result of alloy-induced disorder, the electromagnon is relatively insensitive to Zn substitution, increasing in energy without widening. The results demonstrate that electromagnons and dynamic magnetoelectric coupling can be maintained even in disordered spin systems.