The 6th Circuit Court of Appeals in the US has confirmed that sampling even three notes from a sound recording can violate US copyright law.
The case involved two record companies and two publishers who sued 800 defendants for using samples without permission in various rap recordings. The court ruled that the samples (even the short ones) were valuable, which is clear from the fact that the defendants sampled them and used them in other recordings. In the court's words 'Get a license or do not sample."?
Case at http://www.shorl.com/buralynylene