[UPDATE: Canon has posted Firmware Version 2.0.4, which is intended to address the below-described issues.]
“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 e-mail 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.”
The problems appear to be tied to the lack of audio recording capabilities in Custom modes.