Mac not going to sleep after iTunes stops playing
Check if really iTunes is causing this by opening a terminal and entering:

 pmset -g


If sleep is prevented by a process its ID is listed, like here:

 ...
 sleep      0 (imposed by 129)
 ...


Then check which process blocks going to sleep, in this example:

 ps -A | grep 129

   129 ??        37:39.06 /usr/sbin/coreaudiod


Then try killing (quitting) the process and check if you can send your computer to sleep, and if iTunes is still working. In this example it would be:

 sudo kill -3 129




Otherwise you could try re-installing iTunes, and/or repairing your file-premissions in Disk-Utility.