I'm trying to create an effect that sound like audio 'leaking' out of heaphones - like you are in the same room or nearby someone listening to music (loudly) on headphones..
I'm still learning myself, I'm in an Sound Design class, and it sounds like you want to take down the Highs and even more so the Lows. Just mess around with EQ till you get the sound you like. Have someone put on some headphones with volume turned up and listen to what that sounds like and then try to recreate it in your audio program. Hope that helps.
Yep, as Ian mentioned, you can basically use EQ to knock out the low end and some of the high end. Here's one way--
Apply some compression before you EQ.
Maybe add some distortion.
Hard cut on any frequencies below 700-800Hz
Softer cut on any frequencies above, say 8000Hz or so.
Some mid boost around 2000Hz, maybe a bit higher.
Is there a (moveable) bandpass filter in your audio editor? That might be easier.
On the other hand, what is the headphone-wearer listening to? If it's the stuff those dang kids are listenin' to these days, you might want to do just the opposite...have some heavy bass leaking along with some tinny high freqs.