The best way to save power AND play music is to just turn off the display

• Hibernate will not work. Hibernation means your computer saves its current state to the hard disk, and shuts down completely. You can't play music if your computer is off -- in the same way, you can't if it's hibernated.
  
  
  
  • Ubuntu actually suspends the computer when you close the lid, which is slightly different than hibernate in that it uses just a little bit of power and can resume instantly, but it makes no difference - the speakers, CPU, etc. are all still off.
  
  
  
  

Set Ubuntu to "Do Nothing" when the lid is closed, and disable wireless if necessary

• The laptop component using the most power is usually the display; by setting the laptop to "Do Nothing" when you close the lid, it will turn off the display--saving a significant amount of power--and let your music continue playing.

  
  
  
  
  • You can change this from System Settings...Power:
  
  
  
  

  

  
  



• The CPU can use power if it is active, but Ubuntu manages this intelligently; playing music is not a CPU-heavy activity on any recent laptop.

  
  



• If the music is on your hard disk, you can also try turning off wireless networking to save a little more power.