GTA 5's soundtrack is available now on iTunes. The three-volume compilation of original tracks, new tracks from well-known artists, and licensed songs appearing on the radio can accompany you even outside the confines of San Andreas.
The Music of Grand Theft Auto V's first volume compiles the game's original music from artists like Wavves, The Chain Gang of 1974, and Tyler, The Creator. The second presents its original score--normally it dynamically reacts to player actions across missions and wandering around the world, but here it's layered and arranged into a sort of mult-track overture.
Finally, the third volume includes an eclectic selection of 19 tracks from GTA 5's 17 ample radio stations. You can check Rockstar's Newswire for a complete track list.
The collection is available as a whole for $25, or $10 for each individual volume.