Display sleep but the computer does not even already configured in Energy Saver

I'm using Macbook Pro 13'' late 2011 and Mac OS Mountain Lion 10.8. In Energy Saver settings, I configured the sleep mode as below.

The settings will put the display into sleep after 1 minute idle time and then 1 minute after that the computer will sleep too so the computer should go into sleep after the machine has been idled for 2 minutes. In my case, the display did sleep but the computer did not. If the computer is in sleep, the Sleep Indicator Light (SIL) is on and off slowly but mine is always on. You can check it by manually putting your computer into sleep. The SIL is at the front right bottom of the macbook. Please see the image below.

The cron job I set up and Boom app (boosting sound) are the culprits. The computer would not sleep if the cron job is running so i disabled it (crontab -r) and quit Boom app then it worked.

To check if there is a process preventing your computer from sleeping, run the following command:

$ pmset -g assertions
9/16/15 12:22:30 AM GMT+ 
Assertion status system-wide:
   PreventUserIdleDisplaySleep    0
   PreventSystemSleep             0
   PreventUserIdleSystemSleep     0
   ExternalMedia                  0
   UserIsActive                   0
   ApplePushServiceTask           0
   BackgroundTask                 0

Listed by owning process:
  pid 2631(helpd): [0x0000000c00000630] 00:01:00 BackgroundTask named: "com.apple.helpd.sdmbuilding" 
  pid 2578(installer): [0x000000010000062f] 00:01:18 PreventUserIdleSystemSleep named: "Installing Software (com.apple.installframework.Queue)" 
 Details: Installing ZipCloud
  pid 2581(installd): [0x0000000100000635] 00:00:19 PreventUserIdleSystemSleep named: "Installing Software (com.apple.PackageKit.installd.commit)" 
 Details: Installing "ZipCloud"
  pid 236(coreaudiod): [0x0000000100000638] 00:00:07 NoIdleSleepAssertion named: "com.apple.audio.'AppleHDAEngineOutput:1B,0,1,1:0'.noidlesleep" 
  pid 236(coreaudiod): [0x0000000100000637] 00:00:07 NoIdleSleepAssertion named: "com.apple.audio.'BoomEngine:0'.noidlesleep" 

The value of PreventUserIdleSystemSleep is 1; it means that there is one process preventing your computer from sleeping. Its pid is 236 ('BoomEngine:0'.noidlesleep).