The .AU files are the data files that belong to an Audacity project. You should not touch them - let Audacity handle them.
Audacity works in its own “project” format. There are two parts to an Audacity project - an .AUP file and the associated data. When you “Save” a recording, an AUP file is creates and a folder which has “_data” at the end is created. The data files (.AU files) are copied by Audacity into the _data folder.
To create a normal audio file you must “Export” from Audacity.