The absorption spectra in the P-3(1) <-- S-1(0) range and those of the prompt and long-lived emission components as well as their decay times are recorded for Hg atoms isolated in CH4, CD4, and mixed CH4/Ar matrices at T congruent-to 12 K using the dye-laser excitation. The results are compared to those obtained in the previous work for Ar, Kr, and Xe matrices. The rate of the P-3(1) --> P-3(0) intramultiplet relaxation is only slightly increased, while that of the P-3(0) --> S-1(0) emission shows an important increase due to the coupling to internal modes of methane molecules. The evidence of this coupling is the vibrational structure of the emission spectrum involving nontotally symmetric mu2(E), nu3(F), and nu4(F) normal modes of CH4 or CD4 molecules.