The Canon HV20 mini plug audio input has a very limited useful range, but you should be able to adjust your setup to get acceptable results.
Just on a side note, you can get a wired lavalier mic at Radio Shack for $15 that could make the problem go away, or at least eliminate a lot of electronics ... but the cable will only be about 15 feet long ... you could use an extension, just make sure it's mono.
Couple notes: One is that the mini plug input on the HV20 is a stereo microphone input, and you're plugging in a mono source. This will not cause distortion, but is not ideal ... a simple stereo adapter to break down the input (mono mic into one side of stereo adapter, stereo adapter into camera) would optimize the situation.
Another thing is, you have at least two volume controls that absolutely MUST be adjusted correctly ... hopefully one of these is the problem and soon will be the solution.
I don't know the Sennheiser wireless set in particular, but it probably has a volume control on the receiver box (which might be over by your camera) .. the other is actually IN the camera software ... go through the menus and see if you can select audio input volume and try the "auto" setting, and also try the "manual" setting, and when on manual, turn it down. This is the audio input volume control for the camera itself. It is the level that corresponds to the small level meters you can see in the camera's LCD screen ...
Good luck! You are not alone with these issues ... the beauty of modern video cameras is in their small size, but it's also a problem because you are not provided with proper audio inputs - you have to deal with the stereo mini plug input, which is less than ideal.
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