There was a small audio bug in the just-released 2.0.3 firmware update from Canon. Here is the link to the 2.0.4 update that resolves the issue.

http://web.canon.jp/imaging/eosd/firm-e/eos5dmk2/firmware.html

"There was a phenomenon in which the manual recording settings made in C1/C2/C3 were changed and sound could not be recorded if manual sound recording was used in the C1/C2/C3 settings and the camera was subsequently turned off (or if Auto Power Off was activated). This does not occur in modes other than C1/C2/C3."

You do not need 2.0.3 to update to 2.0.4. You can update from any previous version.

Thanks!