Canon has issued a firmware update for the EOS 1D C. Firmware version 1.3.5 “Fixes a phenomenon in which the line-level audio input function of some EOS-1D C cameras does not operate as expected with Firmware Version 1.3.4.”
According to Canon, The phenomena described above will occur in cameras running Firmware Version 1.3.4 whose sixth digit from the left of the serial number is “1”, “2”, “3”. Examples of affected serial numbers are: “xxxxx1xxxxxx”, “xxxxx2xxxxxx”, “xxxxx3xxxxxx”.
The Canon 1D C firmware is not user upgradeable. The camera must be performed at a Canon Customer Support Center. You can find more information on how to upgrade your Canon EOS 1D C free of charge here on Canon’s website.