For those that don't want to sync audio in post you may attempt to plug a microphone (like a Rode VideoMic, LAV mic, or Wireless Mic Receiver) directly to the camera's 3.5mm mic input, and then adjust levels from within the camera (unless you're stuck with AGC). One problem is that the camera's internal preamp is known to be fairly noisy if you have to boost the signal. The solution for better audio is to use an external preamp before feeding the signal back in to the camera and turn the camera's internal levels down to almost zero. The new Canon 5D Mark III now has a headphone monitoring port, but does it still suffer from internal preamp noise?
So here's an interesting question that comes in that might get your wheels spinning. Can you use something like the Fiio E11 portable Headphone Amp to boost and adjust your microphone levels? Unlike the other Fiio headphone amp options, the E11 has a 10 hour rechargeable battery and volume (gain) control. It's not going to offer phantom power to a microphone, but if you're already using a self powered mic, then there's no need for phantom power. The E11 has standard 3.5mm input and output connections.
We've seen a workflow where some people feed the headphone output of a Zoom recorder (for dual audio and because the preamp is better in the Zoom) back into the camera with a Sescom cable. Considering that option it sounds like a Fiio E11 could work out as an inexpensive external preamp for DSLR video (assuming the output signal is better than the DSLR). Anyone attempt something like this before? The E11 has some interesting specs, and you can take a look at the Product Description (click here).
Fiio E11 Portable HeadPhone Preamp - via Amazon
You can also find these Fiio E11 Headphone Amps via eBay (click here)
Fiio E11 Portable Rechargeable HeadPhone Amp - via eBay