Amazon currently sells audio adapters that enable visually impaired Kindle owners can have their audiobooks read aloud to them. Normally, this adaptor costs around $US20 ($28), but you can make your own for as little as half that. As German ereader site ALLESebook explains, the Kindle audio adaptor is a plastic case around a micro-USB to full-size USB adaptor plugged into a USB-to-3.5mm headphone adaptor. If you're willing to wait a while for shipping, you can find audio adapters and USB adapters for around $3-6 altogether. Once plugged into your Kindle, the device should start to read everything aloud automatically. Keep in mind that this is an accessibility option for the blind and visually impaired, so that means everything, including the menu options, will be read aloud. This may not be the perfect option for someone just looking to turn their regular ebooks into audiobooks, but it's a handy way to save some cash either way.
DIY Kindle Audio Adaptor, Voice View Demo [English] [ALLESebook via The Digital Reader]