I found out that the file is there, and i actualy have read/write permissions. It is also not locked.
The modified date does change to the current time when i change something in the Midi-Audio-Setup menu.
So everything seems fine with this file, eventhough it looks corrupt when i open it with TextEdit. (It looks fine in the preview-tab of the Information Window)
However, when i set a Standard-Output Device, it is reset to the system's choice when i do a reboot. :-(
Not sure what is happening, but would like to point out:
1) The file you mention is NOT a text file, it is a binary preferences file, so it is quite natural that is looks like gibberish in TextEdit.
If you have XCode, and double-click this file, you are likely to see it as it is meant.
It contains a Root dictionnary, with a few items (mine shows 8, but it will vary).
QuickLook must have an interpreter for this type of file, and so is capable of interpreting it and showing it as if it were an actual text file with xml code.
2) The fact that you don't seem to have a /Library/Audio folder is unexpected. When I make changes in Audio Midi Setup I can see the changes affect the
/Library/Preferences/Audio/com.apple.audio.SystemSettings.plist file. Your problem seems to be that this file does not exist, so the system reverts to the defaults all the time.