The problem manifests itself when the 5D Mark II is set to the C1, C2 or C3 positions on the mode dial, with manual audio recording and the levels changed. If it is then power-cycled or automatically powers off, on resuming the camera is no longer able to record audio.
For the time being, EOS 5D Mark II firmware v2.0.3 is not available for download. Canon is recommending customers who have already upgraded to the newer firmware version not to use manual audio recording in the C1, C2 or C3 mode dial positions. The company has stated that it is currently preparing a new firmware update that will replace the withdrawn version, and correct the audio issues.
Canon USA Inc. first issued the firmware update three days ago, and has separately published the following statement on its website:
"Thank you for using Canon products.
We have learned that some users of the Canon EOS 5D Mark II digital SLR camera are experiencing issues with Firmware Update Version 2.0.3.
We are working on a new firmware update to address these phenomena, which will be available soon. If you have not registered your EOS 5D Mark II, please register so we will be able to notify you via email when future firmware updates become available for download.
Canon always strives to provide the highest quality products to our customers. We sincerely apologize for any inconvenience these phenomena may have caused. We appreciate your kind patronage and support."