Help ho,
You'll need to use a different program...MovieMaker CAN do voice annotations
and theoretically record you singing, but those files are saved as .wma
format not MP3. I suggest a program such as SoundForge which I use..or I
Googled and found StepVoice Recorder (which is free to try and only 14.99 to
buy.)
You could also use Windows Sound Recorder and a microphone and then
translate the outputted (saved) .wav into an .mp3 with a 3rd party piece of
software...
Google the following phrase ".wav to .mp3" to find a bunch of software
options, some free, some not....be sure that the software is capable of going
from .wav to .mp3 and not just from .mp3 to .wav which is the more common
usage of such software (for burning cds). Hope that helps! Good luck!