Three short questions:
1. Do you have any "audio mix" files that work normally? If you do, what is the
suffix on those files; could be ".aiff", ".aac", ".wav", ".mp3" or something else.
2. On what app do you normally play those files?
/Volumes/SANS\ TITRE/STEPH\ IFF
It appears that you have a directory/folder named "SANS TITRE" (Eng="Untitled"), is that correct? The "\ " is bash's attempt to designate that the following character is NOT to be interpreted as part of a command nor a flag or a space between flag or other terms. It is to be used as simply a space between two parts of text. It can easily confuse operating systems that did not allow spaces in file or folder names.
Inside that "SANS TITRE" directory/folder, you have a file (or folder) named "STEPH IFF".
3. Is that the name of an "audio file"?
There
is a quite old suffix, "iff", that is/was used by Amiga computers, but it should still have a dot (period), as in ".iff", to designate the suffix. Of course, I doubt any Mac app will 'know' what to do with an "Amiga Interchange File".
"AIFF" is a high quality audio format. I wonder if is ".A" somehow got left out of the name.