I'm not sure what the steps you're taking to do the audio paste. But if you have something audio copy from another app, you should go to the Manage sounds page (Setup->Manage Sounds) and select "Audio Paste". It should prompt you for a filename, and after you have set one click "OK" to have your sample available for Reactable Mobile.
I tested it on our devices here and it works. Can you tell me what are the steps you're following to get this error?