Does playing an MP3 takes significantly more ...

It depands on your definition of "standard software".

While playing MP3s, your computer is constantly reading the file from your HD (HD got a motor inside that takes energy) decoding the file (thru the CPU, more energy) and then outputing the sound to the speakers (the louder you hear, the more energy it takes).

 If you compare it to, say, running Notepad or Solitare that got no sound, rare HD access and little CPU use, then yes, MP3 playing takes much more energy.

I agree with Rattlehead on the previous answer. 

I would just like to add that if the MP3 in question is on a CD, it will eat the battery life with the speed of the gods.

In my opinion, the HD part of reading an MP3 is not that significant - an average song would take 3-4 MB of space, and is read to the memory once every 3-4 minutes (Average length of song), so its not as bad as my preceders answered.

About the CPU usage, you should keep in mind that MP3 files were designed to work on 100 MHz computers (yes, those antique P1 processors), and should not take much more than any "regular" program.

Just follow your CPU usage monitor and see how much it takes to simply move your mouse around... With my E6300 processor it can take as much as 20% of CPU capacity if you move it around real fast... that's approximately 300 MHz (sounds farfetched, but I just checked it...)