Use headphones instead of speakers, or at least turn down the speakers volume. The problem is that the microphone “hears” what speakers say and send it back.
Sometimes there are even echo when you are using headphones. It could be caused by settings or type of your sound card.

Click on the speaker icon bottom right in the toolbar or start the program sndvol32. (Button Start bottom left. Run … sndvol32 OK)

Now you can adjust the sensitivity of the microphone, headphone volume and other settings of your sound card.

When the microphone is already amplified to the max, you can try to enable the boost (20 dB boost), which significantly amplifies the microphone. Now you have to show the “slider” to adjust the microphone.

To do so, in the top of the main menu select Options→Features(Settings), it opens a new window in which you check the Mic or Microphone and press the OK button. Now should be added “slider” marked the Mic or Microphone.
That's not all. Select Options → Advanced Settings at the main menu. Now there appears a new button Advanced under the “slider” Mic or Microphone.
Click on it and a in new large window select option 20 db Boost (or e.g.: +20 dB gain) and save your changes by clicking the OK button. Now test your microphone again, whether the sound volume is already OK now.

(Note: There is a possibility that you cannot find the +20dB Boost choice - some sound cards does not support this option.) Instructions for setting up the microphone boost in Windows 7 from the Odorik.exe program.

Dial the number *6 and you should access the echo mode. You should hear yourself in sufficient quality and volume so you can test your microphone and connection quality.

Your problems with sound can also be caused by the quality of your internet connection.