A couple thoughts. First try resetting the SMC Intel-based Macs: Resetting the System Management Controller (SMC)
and PRAM About NVRAM and PRAM
IF those don't correct it try a different brand of disc, macs can be picky. You can try a cleaning disc that you can get at places like Radio Shack.
if all of that fails you've probably got a failed drive, DVD's work because DVD and CD use different lasers.