Can't access certain songs in I-tunes.

When using I-movie, I can not use MANY of my I-tune purchases. I attempt to load a song into I-movie...it loads but then...I-movie will not allow it have any audio. The name of the song appears in the edit area, but NO audio is available. I did change from SNOW LEOPARD to MAVERICK and that DID give me different version of I-movie. Ever since then, I can't use those files....