Comment 6 for bug 1393650

After some more experimentation, I believe I have repaired the problem of

MATE Menu > Places > Home Folder (or Desktop or other location, like another filesystem)

opening the selection in Audacious.

First I note that even on the affected machines, opening a folder in the file manager worked correctly: that is, double click on the folder or opening a folder from right-click menu > Open opened the desired folder in the file manager window.

What I did that fixed the MATE Main Menu behavior: Opened file manager, then using any folder: right click > Open With > Other Application > Caja fixed the problem in the MATE menu > Places described in the first paragraph. (there were two Cajas to choose from: one with a folder symbol, the other with the file cabinet symbol. I selected the latter).

Yes, Audacious seems to be set (or set itself) as the default application for folders after installation, but for some reason that was ONLY in the MATE menu (called Main Menu in the panel items list). Rechoosing Caja in the file manager as described above fixed the MATE menu as well.

Other comments:
This repair corrected the problem, but not the cause.
I discovered that the MATE Advanced Menu was not affected.
I can't explain why one machine of four was not affected.