Go to Yousendit.com, where you'll be asked to enter your email address in the "From" box. This lets the recipient know who originally sent the file. It is also Yousendit's way of verifying the request. Below "From" is the "To" box, the space for the recipient's email address. Fill in both boxes.
Click on "Select File" to browse your hard drive for the file you wish to send. The Yousendit free account allows you to send up to 100 mb at a time, which should be fine for most audio files.
Click "Send" to begin uploading and sending your audio file. Uploading larger audio files can take some time, and you must keep your browser open until the process is complete. Once it is, you'll receive an email notification confirming your file was sent.