What exactly is the source?
Is it a music file…or sounds, speech…what?
What do you want to use it for? Why does it have to be 1MB? As the others have pointed out, the quality will go down the more you compress it. If you can, redo the conversion from the original to your target size. If it sounds really terrible, then there is really nothing you can do about it.
Some ringtones however seem to manage to sound acceptably terrible at rates of 32kb/s
Maybe do some reading here http://ekei.com/audio/