Well, my setup is that my 3DS is connected to the computer normally, and the audio cable connects into the 3DS' headphone jack, which I attach to the noise filter, then to my computer's headphone jack. That fixes the screechy problem, but now i'm not getting any sound coming from my 3DS trying to record.
edit: I tried flipping the setup (connect audio cable to 3DS and noise filter enters my computer), still not getting any sound from the 3DS (although I am getting background sound effects on occasion). That's my main concern here, making sure I can hear that. I want to be able to get sound from the 3DS and narrate simultaneously.
edit 2: If that's not clear, here's a picture of how I have it set up. http://i1328.photobucket.com/albums/w53 ... 4136ea.jpg
In case you can't see it, the green cable is the sound cable and the black one with the box is the noise filter. Behind my computer is just the cable that I have to connect into my 3DS.
How does it sound without the noise filter? (just an audio cable straight from the 3DS to your computer). You shouldn't get any screeching sound, it's only a mild buzzing noise that the filter removes. If you can hear your 3DS without it, then it may just be a bad filter.
You're sure it's plugged in to the right port on your computer? Do you have the recording volume turned up? The 3DS headphone jack can be very stiff sometimes, make sure the cable is pushed all the way in.