Hi,
Are you using an absolute path when you create the file name with the expression?
The Dynamic Audio prompt requires a relative path. You usually just put the wav file into the Audio folder of the VAD project, and then the wav file is deployed to your server and you just need to create an expression with the name. If you are locating the file somewhere else, try to reach it using a relative path expression, for example: ..\..\..\MyAudioFile.wav
Let me know if you make it work like that.
Thanks,