A somewhat more obscure topic, but hopefully of interest to some. You may have some (musical instrument digital interface) midi files recorded on a synthesizer or downloaded from the Internet; these are music files that the computer (or synthesizer) plays and are essentially like sheet music. In order to play these on a CD player or mp3 player, they need to be converted to sound files (mp3, wav, or others).
There is software for doing this, but an online one that I've used is on hamienet.com. Unfortunately the site launches some "pop-up" advertising, but you can just close those windows. The process for creating an mp3 from a midi file should be fairly evident from the instructions on the page. It basically involves uploading the midi file by clicking the Browse button, then setting the options (or leaving the options as they are), and clicking Convert File. After it converts the file, it gives you the option to download it as an mp3, which can then be put on your music player or burned to an audio CD.