The reason for this, is that there is no file area associated with that custom profile field you are adding. So Moodle would not be able to store the recording. There is a whole lot of extra plumbing that goes on behind the scenes to store files, and its not done there. Al's method should work for you in the meantime though its not as neat as you would like. If you use Cloud Poodll for Atto that would also work because the audio is saved in the cloud, not in moodle.