Wow! Thanks for the education, gwen-dragon and sgunhouse! I didn't know that "about:plugins" existed. I still need more education about what MIME type has to do with access to plugins. I will respond to gwen-dragon's question first.
In the first place, I have 8 plugins, and these are the results: VLC Multimedia Plug-in (libvlcplugin.so) (twice) (both plugins crash), Kaffeine Starter Plugin (kaffeineplugin.so) (twice) (both of these work, but the "seek slider" is inoperative, making the app useless), Shockwave Flash (flash-mozilla.so) (Not Applicable), Skype Buttons for Kopete (skypebuttons.so) (Not Applicable), OpenOffice.org Plug-in (libnpsoplugin.so) (Not Applicable), Shockwave Flash (libflashplayer.so) (Not Applicable).
One thing that really seems strange to me is that Firefox doesn't use the VLC plugin, even though in Synaptic the plugin is called "mozilla-plugin-vlc"! That version is "1.1.3-1squeeze6"--I wonder if that is too old for either Opera or Firefox to accept!? Also, when I do a Firefox search for the VLC plugin, it gives me a jungle of plugins, but very few that appear VLC related. The VLC that I see available for Linux for download is version 2.1.5.
Next, sgunhouse, you asked about "Which plugin is listed for that type in Preferences > Advanced > Downloads?", Well, in the first place, there is a check mark for "Hide file types opened with Opera". There are a total of 172 items listed! Of these, 7 are blank in the "File Extension(s) column. There are 5 in the mp3 category. Note that 2 of these included other than mp3. The MIME types are: "audio/x-mpeg", "audio/mp3", "audio/mpeg3", "audio/x-mpeg3", and "audio/mpeg". When I removed the check from "Hide file types opened with Opera", at 200 I was about half way through, so I assumed 400 and stopped counting! There were 28 with a blank "File extension(s)", and there were the same 5 in the mp3 category.
I've noticed some remarks during my Internet searches about the proprietory aspects of mp3. If that's the problem, I wish some one would just let us drop a couple nickles in the slot and let us get on with it. But, of course, that makes no sense, because Knoqueror has no problem, nor do you, gwen-dragon, with Debian 7.8.
What else do I need to look for? Thanks