Easily done using Audacity! Many fine tutorials for this exist on YouTube, just search for "Audacity" and you'll come across several. This software is free for both PCs and Macs. You can open ,mp3 or .wav files or a few other formats, and record in any of those.
Midi files are "sheet music for computers." These aren't sound files, but instructions to a sound card or software on what notes to play, for how long, how loud, tempo, and so forth. Here's a recommendation for the "Midi2Wav Recorder" software that I use!
Midi2Wav Recorder is a software for Windows that enables you to convert MIDI to WAV, MIDI to MP3 and MIDI to WMA files. The conversion is as easy as one touch of a button. The software provides two ways to convert MIDI files: Soundfont Rendering and Soundcard Recording. The first method uses external soundfonts and makes conversion much more faster than real-time recording. It provides soundcard independent and silent conversion. Alternatively the second method allows to record sounds synthesized by your soundcard during MIDI playback. Using Midi2Wav I have easily converted midi files to wav and mp3 file and had a lot of fun listening to music with instrumentation not used by the composer, such as creating music box tracks from midi files of Mozart's music (great for lullaby type of music!). Reach the vendor of this software at http://www.midi2wav.com.
- Change instruments played by each midi track
- Control track volume
- Transpose MIDI notes and key
- Use external soundfonts
- Change tempo and duration
- Control audio quality
- Use batch conversion for large sets of midi files