This solution was posted on the following page:

From http://www.goteamspeak.com/index.php?page=faq

Q: My mates tell me I produce echos. How to fix?

A: There basicly are two ways echos can occur: Sound from your speakers/headphones being recorded by your microphone. Your sound driver recording all sound that you hear for you. Now, 'which one do I have' you might be asking yourself… You can find out quite easily: Just turn your speakers/headsets off (volume = 0%), and ask your mates if you are still echoing. If you are not, you got (a); if you are still echoing you have (b).

Fixing (a):

This type of echo is nearly impossible to fix completely, but you can minimize it greatly if you follow a few general points: Switch your speakers off when TeamSpeak is on, use a headset to hear. Use headphones that shield well, meaning they prevent as much sound as possible from being audible to the outside. Try and keep your microphone as far away as possible from the headset (or more general, the sound-source). Lower the volume of the sound-source. Fixing (b):

These echos are produced by your sound-drivers, they should be configurable by your driver too (its not a bug, its a feature). From windows users We have often heard you can select from which devices to record in the mixer (a mixer is the place where you can slide all these volume controls). Make sure the microphone is the only entry that is being recorded from. How exactly this is called, and where exactly you can find it depends on your driver/OS combination, you just have to search around. Good luck.

Alternate Solution:

Dealing with the TeamSpeak "Echo" problem:

• Step 1: Doubleclick the speaker ("Volume") icon in your system tray. This brings up your "Play Control".
• Step 2: Select "Mute" for "Microphone" and then select "Options" > "Properties" > "Recording" and make sure microphone is selected in the device list, then hit "OK". This brings up your "Recording Control" dialog and you should make sure "Microphone" is selected.