Yes, contrary to popular belief and practice, sampling of an original copyrighted song without permission of the copyright’s owner is illegal copyright infringement.
Unauthorized sampling actually violates two potential legal rights. First, the instant you sample a portion of someone’s song (no matter how small), it constitutes a violation of the copyright in song itself - the © symbol - which is owned by the song writer or the music publisher. Second, sampling violates the sound recording copyright - the symbol - which is usually owned by the record company or recording artist. Thus, sampling without prior permission subjects the illegal copier to a copyright infringement in federal court by the original author (or publisher) and by the record company.
(Reprinted with permission of Ruben Salazar, Esq.)