Canon has released a firmware update to fix 
an issue some EOS-1D C users were having with the latest features.
Firmware Version 1.3.4 brought line level audio support to the 1D C, however there were some issues with a few camera bodies regarding this feature.
Information taken from the Canon website:
Details
Firmware Version 1.3.5 incorporates the following improvements.
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.
Affected Product
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”.
Support
Canon USA will perform Factory Updates on the EOS-1D C free of charge. Shipping and handling charges may apply. Please contact the Canon Customer Support Center for details.
This information is for residents of the United States and Puerto Rico only. If you do not reside in the USA or Puerto Rico, please contact the Canon Customer Support Center in your region. Contact Information for Inquiries
Canon Customer Support Center
Phone: 1-855-CINE-EOS (toll free)
1-855-246-3367
TDD: 1-866-251-3752
Email: [email protected]
For additional support options: pro.usa.canon.com/support
via/CanonRumors
