DSLRs aren’t designed for pro video and one of the important features you give up is the ability to monitor audio. That is, until Christian Sundsdal figured out this DIY work around to monitor audio from his Rode Videomic.
For only $37 in parts, he was able to rig up a jack to monitor audio from any 3.5mm microphone.
“I have found one simple workaround that really works.And a bonus with my setup is that you can connect two mics. With my setup, i have a wireless clip-mic, and the videoMic connected… works great, and i can monitor both with headphones.”
See a diagram of the setup here.