Robert Rodriguez was hired by Tarantino to score the film. Rodriguez did it as a favor for his friend, Tarantino, asking to be paid one dollar for his work. Tarantino repaid this favor by directing a scene of Rodriguez's Sin City for the same fee. RZA said of the soundtrack:
"With Kill Bill I did score and songs, meaning that we put a lot of songs in [the movie] from old collections of records and I composed music for some scenes, natural music. When we did Kill Bill 2, you know, we brought Robert Rodriguez in. Check this out, he took my music and he kept the foundation there, though. With Robert he didn't want to remove any of the electronic [sounds]. He said 'No.' I was like 'Take out all the electronic stuff, you know, so it can be [more like a traditional score].' He said 'No, man. I like the electronic stuff. This is the reason I wanted to do this.' So he took the electronic stuff and kept it there, then built the orchestrations on top of it, you know what I mean?"
[IGN]"With Kill Bill I did score and songs, meaning that we put a lot of songs in [the movie] from old collections of records and I composed music for some scenes, natural music. When we did Kill Bill 2, you know, we brought Robert Rodriguez in. Check this out, he took my music and he kept the foundation there, though. With Robert he didn't want to remove any of the electronic [sounds]. He said 'No.' I was like 'Take out all the electronic stuff, you know, so it can be [more like a traditional score].' He said 'No, man. I like the electronic stuff. This is the reason I wanted to do this.' So he took the electronic stuff and kept it there, then built the orchestrations on top of it, you know what I mean?"
[Wikipedia]