Are you using Windows Media Player to listen to your midi and mp3 files? Most PCs will use this as the default program for music files and double-clicking on the file itself will usually result in WMP opening automatically. There are some options which are very useful to singers.
Slow practise is good practise and in WMP you can control the speed of playback without altering the pitch. Here’s how. Open the View drop-down and select Show Enhancements

Next, select the speed settings About 0.8 is ideal for speech. You can control the quality of sound by selecting the other Enhancements.