Your audio amplifier may also be directly picking up your signal and feeding it back to you. Try turning off the amp and using headphones until you get things adjusted, then try the speakers again.
Also, If you haven't already, please watch these two excellent videos about setting up your TX audio on PowerSDR. They will save you a lot of grief.
These are excellent tutorials.
I would recommend setting your mike for "Wide" audio and then setting the TX EQ and TX filter to get the response you want. Look at your TX audio spectrum and make sure you do not have the low frequencies way too high. They should not be overpowering the other frequencies. work with the TX EQ to shape your audio response and be careful to follow these tutorials about proper gain settings at the several different places, and then never run your DX processor too high, it gets nasty pretty quickly. Frankly, I hardly ever use it, but i DO have the COMP set at about 6 on my 1500. These rigs are capable of fantastic audio, but you have to work at it i bit to get it there. Then SAVE, SAVE, SAVE your profile! Then make another one so you have one for rag chew and a different one for DX/Contesting/Weak signal, etc.
Good Luck! you will get there.
In addition to getting your audio levels correct, the next most important thing is to spend time getting your EQ correct in the 10 band graphic equalizer. This will make or break your sound. It can turn a relatively inexpensive mike into a pretty good sounding one, or it can take a $400 mike and make it sound like garbage. Many people run the bass WAY too high and it sounds muddy or boomy. Others cut or boost the midrange so poorly that it either sounds honky or like you are in a hole. Failure to have adequate signal strength in the 1K, 2K & 4K zones will make your signals hard to understand. Too much will make you sound like you have a lisp or a whistle when you talk.
The key is to listen to your signal in a 2nd receiver, preferably by taping it and listening to it later. Using Monitor and headphones will not give you a realistic impression of your own sound because you will hear it as much through bone conduction as through your ears. It takes a little bit of time with a dummy load and recorder, but the payoff will be constant unsolicited compliments in your audio!