If you are using Skype and hear your own voice, this is what Skype calls an "echo." Skype has a built-in echo canceler, but sometimes it does not work. If you hear echoing from your computer's speakers, the problem lies with the person on the other side of the call, even though it's your voice that you're hearing as an echo. You can make a few adjustments to address this.
Background Noise
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For Skype's built-in echo canceler to work properly, there should be little background noise on either side of the call. You should both be in quiet locations.
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Speakers
You are hearing the echo because the other person's microphone is picking up your voice through her speakers. Tell her to turn her speakers down or, if possible, to move her microphone as far away as possible from the speakers. Ideally, the microphone and speakers should be at least 8 inches apart.
Skype Updates
For optimal call quality, you and the person on the other side of your call should have the latest Skype updates installed. If you're not sure if they are installed, click the "Help" tab and select "Check for Updates."
Headset
If none of these measures work, ask the person on the other end of the call to use headphones to hear you. This will eliminate the problem of her microphone picking up your voice.