What is the difference between a musical work and a sound recording?
A musical work is the composition of a song (the actual melodies and harmonies), the arrangement of the instruments (what parts the guitar plays, etc.) and the lyrics, whereas a sound recording is an actual recording. Thus, the author of a musical work is the musician who composes the music and writes the lyrics, whereas the author of a sound recording is the musician who actually records the music (or possibly the record label producing the recording).
So CDs generally have two copyrights for every song - one for the musical work, and one for the actual sound recording embodied on the disc. Thus, when someone sells a pirated CD, they are actually infringing two copyrights - the musical work copyright and the sound recording copyright. However, when someone makes an unauthorized cover of a song, they are only violating the musical work copyright.
When the issue of copyright infringement relates to sampling, this implicates the sound recording copyright, since a sample comes from the actual recorded musical track. The musical work copyright may or may not be implicated in this situation, depending on the nature of the sample used.