Those manual recording levels have been causing problems for users shooting in custom modes (C1, C2, and C3), resulting in an inability to record audio in certain situations. Customers who have already downloaded the update are instructed not to use the new manual audio controls, and to set their sound recording settings to Auto when shooting in a custom mode until a fixed version of the firmware update can be made available.

From the announcement on Canon's Website:

We recently discovered a malfunction that occurs with Firmware Version 2.0.3, in which the manual recording levels for C1/C2/C3 are changed and the EOS 5D Mark II digital SLR camera becomes unable to record audio if the power is turned off (or if Auto power off takes effect) after registering "Sound Recording: Manual" in the camera user settings. For customers who have already downloaded and updated to Firmware Update Version 2.0.3, when using the camera with the mode dial set to C1/C2/C3, please set the Sound Recording settings to Auto to record with audio.

Canon did not say exactly when the repaired firmware would be released again, but it urges EOS 5D Mark II owners to register their cameras. Registered users will receive an e-mail notification as soon as the firmware is back online.