In first place be sure to have installed 3CX Voice Application Designer, Build 3679.
Then, you need to add a prompt of type DynamicAudioFilePrompt (click on the arrow on the right). Take a look at the screenshot in this blog article from the Spanish blog:
http://www.3cx.es/blog/vad-archivos-dinamicos/
That article will be translated to English soon.
Regards,