Sorry I didn't reply sooner. So, if Im understanding correctly, what you need to do is "activate" your video cameras record mode when a sound (or sound level, etc) is detected at a remote (remote as in not right next to the camera)? then record the sound from the location of the mic rather than the camera? Or will the camera be running constantly?
If you need to trigger the camera remotely when a certain sound level is heard, I should think hacking one of the yongnuo triggers might be the easiest, as long as the camera is set correctly (obviously!) then once it receives a signal (usually 433 MHz with some basic encoding/decoding - like a garage door, or light switch). So if you can modify the transmitter to transmit (i.e. fake pressing the button) which I imagine you can do in a short time, then that's that part solved.
As far as recording audio from some place other than where the camera is, there are BT devices as mentioned, personally Im not a fan of cheapo Bluetooth devices, but may work. However, why not pick up a cheap Dictaphone, locate it near to the transmitter, and have this turn on at the same time as the camera activates via the remote transmitter - not hard to do. That way there is less chance of picking up unwanted sound etc from across the room, also removes a lot of potential issues that are common with cheap wireless/BT (or cheap anything !) Or maybe use a laptop/tablet/phone with a mic ? Then, once you have the video on your camera, audio on your recording device, its just a matter of spending a few minutes getting the timing right and replacing any existing audio that the camera picked up.
There are some fairly cheap RF audio transmitters about, Ive no experience with them though. The only radio mics I have used recently are not what I would call cheap!
As far as power goes, as long as you have enough for your camera - depends of course how long its going to run for - additional battery pack (DIY our purchased), or is a mains input an option?
The wireless triggers I mention ususally use a CR2 or similar 3V lithium battery, the only thing they usually do is wait for a signal, then tell the camera to trigger the shutter (or start record I presume) So unless the camera needs a constant signal from the receiver (i.e. it needs the shutter pressed permanently or similar) then I wouldn't have thought that would be a problem, you can of course easily knock up a higher capacity battery pack if that's likely to be an issue. Likewise with the transmitter, usually a 12V 123 or similar, and again only does anything when the button is pressed - unless, like "B" setting on some cameras it needs to be constantly pressed (or a switch turned on) then that is also unlikely to cause problems, again, easy to get a higher capacity battery or pack.
Hope that gave you an idea or two...
[EDIT] as mentioned, a lot of receivers sit on the hotshoe, easily overcome with bungees, elastic bands,velcro,gaffer tape etc - Im assuming the camera will be on a tripod or clamped somewhere so I wouldn't have thought that would cause a problem.[/EDIT]
Last edited by CalvUK; 20-05-15 at 14:04.
Reason: added a bit