We don't know. You've only presented a problem and provided absolutely no useful information for us to work with.
First, we need to know a few things. What version of Music Maker? What's your computer? What processor (CPU) does it have? How much RAM (random access memory) do you have? Is the graphics integrated or do you have a video card - what is it? Is the audio integrated, or do you have a sound card - what is it? What operating system are you running? What anti-virus software are you using?
Do you have any other programs running when using MMM? If so, shut them down! Sometimes, even the anti-virus software can interrupt the flow.
In MMM program settings, what driver do you have selected?
In addition to all the information Procyon has requested can you explain what you mean by . . . uploaded an mp3 file, and when i try to record over it . . . .
To me this means you have uploaded a mp3 to somewhere on the Internet, for example Youtube, and are then trying to replace it by recording over it.