When recording sync sound with the 5d you really should use a separate recorder like the Zoom H4n. But then you need to sync the sound in post. A slow and annoying procedure. Pluraleyes from www.singularsoftware.com does it all for you automatically.
You don’t need to use a clapperboard or ident or even a sync clap. Just make sure the 5dmk2 is recording audio so it has something to match up to. Best to use a separate mic like the Rode Shotgun to make sure you get the best quality audio for Pluraleyes to use to math up.
Watch this video and see just how easy it is to sync sound with the 5d and separate sound.