Microphone plays in only one speaker?

I have a new HP computer running Windows Vista. I just bought a new sony microphone for recording on my computer but the sound will only play out of one speaker. Ive tried several different mics and they all do the same thing...record and play out of only one speaker. I have spent hours on the net looking for the answer to this seemly simple problem yet no one really has the correct answer. can someone please help me! Its driving me crazy!

Microphone plays in only one speaker?playhouse

You may have the wrong microphone adapter (mono, left only) or your microphone port is defective. Your mini-plug should have three segments for stereo, two if it is mono or only one channel.

Check you Volume Settings to make sure the balance has not shifted to one side.

Microphone plays in only one speaker?plays opera theater

AM-NM centaur's answer is very good.

Also make sure the plug is firmly pushed in, stereo turns to mono if only half inserted
check the BALANCE volume settings