A demo recording of you doing voice-over work is your CV and your business card combined. If you're applying for a voice-over job, you can send the potential customer your demo via the Internet. Ideally you have multiple demos for different types of gigs. For commercials, clients want to hear 60 to 90 seconds of voice work. If you're auditioning as an audiobook reader, five minutes of demo proves that you can stay in character over a longer stretch of time.