Sinrim: If the mp3 plays okay in SONAR - without distortion - but the microphone is the thing that's distorting, then check to make sure that the microphone is plugged into the correct input jack on the computer. If you are using the computer built-in audio capabilities, often the computer will have more than one "hole" that looks like the same size as the microphone jack (probably a 1/8th inch jack). You might have plugged the microphone into the wrong jack. Try the one next to it and see what happens. Sometimes the input jack for the microphone has a little picture of a microphone next to it.
The next thing to try is to find the systems software application that controls the sound interface. Windows/start/control panel/"sounds and audio devices". There you might find a way to turn down the input signal of the microphone, which might be what;s causing the distortion on the mic.
Lastly, look at the track in SONAR where the microphone signal is placed. Are the signal meters going all the way to the top (red?). If so, you may simply need to turn down the track volume for that track within SONAR.
It's hard to completely solve your problem without actually seeing your computer setup, audio interface, and speaker. But, that's the best I can do with the information I've read from you.
I wish your sister a long and happy marriage. I'm sure things will work out fine in any case. Best wishes, -Dave
[edit: okay... I can see that I was way late on this and that Sinrim seems to have made something work okay. Sinrim, your "english" is good enough so that most can understand what you are are trying to communicate - and thanks for trying that. There are foreign born people that live in the United States that don't even try!]