The influence of Ge content on the local vibrational mode of substitutional carbon in Si-rich Si1-xGex single crystals has been investigated by infrared Fourier-transform spectroscopy and ab initio modeling methods. Czochralski-grown Si1-xGex samples doped with boron and carbon have been studied. The Ge fractional content was varied from x=0.004 to x=0.044. To reveal the C-S-related absorption band in the Si1-xGex the difference spectra between carbon-lean and carbon-reach Si1-xGex samples with the same Ge content were studied. We have found that the C-S-related absorption band in the Si1-xGex alloys red-shifts and broadens with increasing Ge content. It has been found that at x > 0.015 the C-S absorption band consists of two overlapping lines corresponding to different combinations of Si and Ge atoms neighboring the C-S atom. The calculations show that substitutional carbon atoms avoid Ge ligand atoms, and should be found in Si-rich regions. These results also reveal that the softening of the C-S mode frequency arises from the SiGe volumic expansion.