ASND files are files generated from Soundbooth, an Adobe application that has been discontinued. From Audition 23.0 onwards, you will no longer be able to open .asnd files in Audition directly.
From Audition 23.0 onwards, use the following instructions to open .asnd files:
Rename the file from <file_name>.asnd to <file_name>.zip.
Unzip the file using WinRAR into an empty folder.
Import the Mixdown.wav file in the folder into the latest version of Audition.
Open Terminal.
Browse to the location of the .asnd file.
Execute unzip <file_name>.asnd.
The contents will be unzipped into the exact location as the .asnd file.
Import the Mixdown.wav file into the latest version of Audition.