Real-time audio processing: Reducing noise and feedback (program or dll)

I have got the following problem:

The signal of some microphones should be amplified without getting too much noise or feedback. I use a normal amplifier already, but if I set the volume too high, I hear backgound noise (the normal one from the microphones). Also, if the microphones get too close to a speaker, I get a feedback effekt ( definition: The pickup and re-amplification of an output signal by its input resulting in an unwanted "howling" sound).

Now I want to send the final amplified sound signal through a computer. There, the noise should be reduced (if possible) and the feedback should be eliminated. Easiest way to stop a feedback effect is perhaps to eliminate all sound above a chosen volume, but I am not sure, if that is sufficient.
Since the microphones are used in a theater play, all this has to happen in real time.

Possible solutions are:
- a program (preferably free, but buying one is also ok) for Windows (best), Linux or Mac
- a dll or something which supplies all functions needed for real time audio processing and allows me to program a filter with delphi or c++

Since I know little about audio processing, I would need a rather easy to use dll...
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JNSTAUBConnect With a Mentor Commented:
have a look on electronic solution with a RC filter and phase shift by googling on LARSEN
this phenomena is the LARSEN effect, it's of course possible to filter it digitally but some rules to place microphones are available at google.
You should place the microphones optimally first, as mentioned, because it will save you a lot of trouble later - the simplest rule is to put them behind the plane of the speakers, so that they don't pick up what the speaker is outputting.  A good program for sound processing is CoolEdit Pro - you can do all kinds of noise filtering with, like eliminating clicks and pops from vinyl recordings.
Valia3Author Commented:
Thanks for your answers.

The microphones are placed behind the speakers, but the setup is not ideal for amplifiing the sound a great deal (since echoes get back to the microphones). Sadly, I can not change the setup. Also, I can not rely on the actors singing loud enough (we tried that already). As I see it now, there is no possible "hardware" solution for the problem, only filtering the incoming audio. It is possible to use some efford and / or money to get a good program and learn how to filter feedback and noise out.

As far as I can see in the demo version of CoolEdit Pro (which is now named Adobe Audition), there is no option to process an incoming audio signal in real time and sending the output back to the speakers / line-out. This is, what I would need.
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