That's pretty impossible to do. Background noise is such an issue that often even professionals have to re-record dialog in a scene if it was shot on a windy day or in a place with too much traffic noise.
It's sometimes possible to filter out certain frequencies of noise, such as an electronic hum or buzz. But you can, for instance, take a recording that was made at a noisy party and amplify one person's voice without amplifying everything else in the room also.
In one of your other "Audio" posts, I linked to an article on attempting to do what you are asking for.
As per Steve G's response, while not impossible, it CAN be very difficult, and success will depend on so many factors. Take a look at that linked article for some ideas. I also think that the greatest chance of success will be outside of PrE.
I have a much younger than me friend that works at Adobe. His product used to be Adobe Audition. To help me figure out what it is, he explained it is commonly used by rock stars. Since that meant nothing to an old pilot, he explained that the NTSB has used it to isolate the pilots' conversation recorded on the "black box" after aircraft crashes.
Anything is possible! Audition is part of the CC for $50 a month!
As we have three, somewhat similar questions, I will Copy my reply from one here, and then Lock that thread.
Amplifying, or attenuating only part of an Audio Stream is not all that easy. You can often get what you want, or nearly so, by using EQ to amplify part of the frequency specturm, while attenuating other frequencies. This takes so experimentation, and also critical listening.
One can gate certain frequencies, around those of the dialog, with Cutoff Filters, say both a Highpass and Lowpass Filter, setting those as needed, and again, by listening critically.
While PrE offers some Audio editing capabilities, I find much of such work better done in an audio-editing program, like Adobe Audition (my first choice), or even the free Audacity.
PS - some of the techniques that I use, are listed in this article on attempting to remove "noise" from an Audio Stream: http://forums.adobe.com/message/2860521#2860521
Having only threads, with one topic, is much easier on all - the OP, and those, who reply. Hope that this does not cause any issues.