I read somewhere that protected audio files are restricted in some way.
Yes, they won't be protected, if there was no restriction ;)
The restriction means: Only people filling out the questionnaire can access the file. They cannot share the link to the media file (which one can retrieve from the HTML source code of the questionnaire page) on the Internet, because it will become invalid after few minutes.
This saves you from the situation where a lawyer comes and says that you have been publishing the audio file on the Internet, although you had only a written consent that you are entitled to publish the audio file as part of your questionnaire.