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 CS-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 CS-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 CS absorption band consists of two overlapping lines corresponding to different combinations of Si and Ge atoms neighboring the CS 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 CS mode frequency arises from the SiGe volumic expansion.