Dalek Voice

Make sure nothing is blocking or covering the microphone (case, your fingers, etc.). If you are unclear as to where it is located, have a look on this page: http://www.apple.com/iphone/iphone-4s/specs.html.

Scroll down to External Buttons and connectors.

http://support.apple.com/kb/TS1631?viewlocale=en_US

Exterminate!!

Do you voice memos sound distorted? Did you check the mic for debris?

may be an issue with your carrier services if your voice memos sound fine

You're welcome.

Does this happen everywhere you go? If not, it could be a poor signal problem at your location or an issue with a particular cell tower.