Q. Can I use voice samples of famous politicians (George Bush, Barak Obama, etc.) in my own compositions?
A. While speeches and sound bites from well-known politicians and other public celebrities may not have been copyrighted, the use of another person’s voice sample in your own composition may require clearance to be properly licensed and devoid of any risk of infringing use. However, a good case can be made that the use of such a public speech in an artistic way is a "fair use" of the speech's content.
As far as copyrighted content goes, one example we are familiar with is Martin Luther King's estate pursuing infringement claims for use of "I have a dream" refrain from one of his major speeches, when it has been used in songs.
In general, be aware that if your track which includes such a sample becomes famous, you may have receive a claim from the person (or their attorney) whose voice you used and you may have to defend such a use as a fair use and may find yourself in a position of having to pay money to the person for the use of their voice in order to avoid a lawsuit. Use such voice samples at your own discretion and remain aware of the potential consequences of such a use.