Generates an mp3 rendition of a captcha code and returns its url. The captcha Code string is case-insensitive, must be alphanumeric and between 2 and 20 characters long. Extend increases retention of the mp3 from 3 to 30 minutes. Obfuscate adds effects to the audio to mitigate speech recognition abuse. Prefix prepends a random phrase to the audio. Count prepends an announcement of the captcha length to the audio. Mode is one of simple, phonetic, both or animal and selects the style of captcha reading.