Music producers, songwriters and composers rely on MIDI technology to help them musically express what they want to in their tracks. MIDI, which stands for Musical Instrument Digital Interface, lets you use an interface to see the music you've created – it's a protocol. So, you can create and record music in a digital audio workstation (DAW), or create music on your piano or guitar, and input it into MIDI software. The music is displayed as messages, or descriptions, of the music notes you've played.
When you've completed a song in your MIDI software and you're ready to share it with the world, you need to synthesize the track to an audio format. Otherwise, it might not sound the way you'd expect. In most cases, you can do this through the audio production software you own. There are other ways, though.
You can find out how to convert MIDIs to MP3s with a quick search online. In fact, you can find free options to convert MIDI to MP3 online. Many of these converters exist on websites such as Zamzar. You simply upload your MIDI file to the website, choose which file type you want to convert to – in this case, MP3 – and then enter your email address to receive a download link. It's possible that these online methods could result in a loss of quality, though.
Another method for conversion of MIDIs to MP3s – or MP3 to MIDI – is through programs that you can download for free. You won't need to go to a website every time you want to convert a file, so it cuts down on time spent converting single files. Some, like Hoo Tech's MIDI to MP3 Converter, support batch conversion to further save you time.
Converting an MP3 to MIDI is more complicated, depending on the complexity of the track. For instance, is it a single instrumental piece, or several instruments with vocals? In this case, you want to use an audio production program that is meant to map out all the different parts and pieces, such as Melodyne. You can use Melodyne in conjunction with a DAW, letting you change bits of the song without degradation. However, you can't expect a flawless transcription. You are likely going to have to go through the MIDI file and make corrections to some areas to ensure it's right.
Finally, a solution that's a bit more time-consuming but cost-effective is to use a free audio editing tool like Audacity to slow down an MP3 so you can easily transcribe the music to notation. This method requires that you have musical theory knowledge and know how to read and write music. Between the free methods, you can experiment and then choose the one that works best for you.