Mainly its a confidence thing if your voice is poor when you have actually been speaking a number of times during that day. It is not uncommon when stressing the voice in a rarely used manner or even for your age normal speach to have distortions in sound either through coughing, cracking, or hoarseness. Sorry but puberty sucks, your voice will probably not be smooth for a few years yet (assuming you didn't hit puberty a long while ago... I am a guy a bit different time scale) When your super nervous, your voice will either be irregular or monotone.
You probably have too much experience with reading versus talking. I had the bad habit of mumbling on top of using higher end vocabulary. People thought I was insulting their intellegience or a show off in that regard. Actually once I began talking more I believe I have better enunciation then a lot of people probably from reading so extensively and from having debate in high school.
The vocabulary thing is just something that you will have to bear with for awhile. Its how you think, you have to get used to mirroring your words to the level/ manner of speaking of the person you are talking to. That takes a lot of time to master if you ever truly do.
So all in all you should practice talking more and not worry about your voice cracking or being rough (lack of sleep can affect it too). Reading is a good start. Talking with your parents and/or siblings more would be a better start or next step, then talking to classmates, and then friends (assuming you have little to no friends). Finally don't get irritated when you have to repeat something. Sometimes it doesn't have to do with how you sound but they aren't paying attention, are to tired to understand, or are so astounded that you actually said something that they missed what you said.