Audacity is software for using sounds. You can record things, and edit them, and then save them and send them to other people. It is free, is available in English, Japanese and many other languages. You can use it with Windows, OSX (Macintosh) or Linux. I use it 3 or four times a week to make an audio podcast (like an Internet radio show). But you can use it in a few minutes to record your voice, or have your students record their voices, and send them to you for homework. Audacity uses lots of different sound formats, but most people use MP3, because it makes small files that any computer (or your iPod) can use.
On this sample here, you can see the buttons that look like a tape recorder. Use them to record and save your files. You can edit the sounds much like a word processor, cutting out parts you don’t like or adding background music if you want to get fancy. You can then use the sound files in presentations like PowerPoint, or to make examples for students, or put up on the Internet for people to download and listen on their iPods.