Legality of Sampler CD?

You have to get permission from two different groups: the record companies who hold the rights to the recordings of the song, and the songwriters who hold the copyrights to the songs. The former can be gotten from the records companies themselves (or from agencies like Harry Fox which work on their behalf); the latter can be gotten from SESAC (if all the bands are licensed through SESAC) or from ASCAP and BMI (or whatever licensing company manages their rights & royalties.)