What OS are you on?
In Windows, you should be able to go to Folder Options>File Type and there just associate the WAV files to Au 6 - probably set to Au 5.5 now. Note: the exact syntax and location of the File Type dialog might differ, Windows version to version.
On a Mac? I do not know where one would set file associations, but others should, and it might even be in the Mac OS Help File.
On the Mac you can do this by ctrl-clicking a file and selecting Get Info. In the Open With section, select the desired application and click Change All.