Will an HDMI to DVI cable between a tv and a cable box interfere with the optical cable for a sound system?

My parents have an HDTV with a DVI hookup and a digital cabe box with an HDMI slot so I suggested that they get an HDMI to DVI cable to replace the S-in cord that they are currently using to get the best picture but tey seem to think that this might interfere with the surround sound system since HDMI cables can transmit sound too. I feel pretty sure that this shouldn't be a problem but I don't want to spend money on a cable unless I'm positive.