In multimedia you can get away with mediocre images much easier than poor sound.  To get good sound you need to go to an external DAT recorder like the Marantz 661 ($600) or the popular Zoom H4N ($350).   Both have internal mics but to get good sound you need to get some good external mics.   I use the Sennheiser MKH 416 or a wireless lavalier for interviews and the Audio Technica 825 stereo mic for ambient sounds. 

The Automatic Gain Control on the 5D Mark II makes it impossible to record a decent sound track.  When recording sound you want your input volume control to be at a steady level.  The AGC raises the volume setting any time there is a period of silence resulting in weird volume fluctuations through out the recording.  Unfortunately this is not a setting that can be turned off.  If you want to get away from carrying a DAT recorder and the extra hassle of syncing sound in post production Beach Tek has a solution to get around the AGC, but it still does not deliver the sound quality of the Marantz 661 (24bit/ 96kHz).  I decided to beef up the 661 a little further by getting it through Doug Oade who replaces the stock 661 mic pre amps for an extra $160 to increase the signal to noise ratio.  His site is a good source for info on field recording.

And finally it is necessary to listen to your sound throughout your entire recording.  A good pair of headphones is a must.   The industry standard is the Sony MDR 7506 ($100).  You wouldn’t take a picture without looking through the view finder, so why would you record sound without monitoring it. 

I recently watched Sound for Film and Television, which I recommend for a good and entertaining tutorial.  If you have any resources you recommend please share!