(In reply to comment #36)
> if everyone is also satisfied, I'd like to go ahead and commit it and move
> on to the detection simplification.
If you're OK I'd still like to try with SVN FFmpeg, since that gave endless
trouble before. I assume you have done this, but since I'm inexperienced there
is a chance I could find a problem by doing some something silly.  

I don't see we can do a "Stereo Tracks to Mono" on an M4A imported via libfaad
as it would take too long. But could or should you do some hackage in our
FFmpeg code so that it imports "first time" as mono? I think the answer to that
is "is this a libfaad bug"? It appears that upMatrix for "implicit signalling
of PS" is deliberate, unless the commenting is the bug. Certainly users would
prefer if mono AACs came in as mono irrespective of decoder and if so we don't
have to release note. Apple accepts mono AAC as part of its standard doesn't
it?     

> I'd like to leave it with FFmpeg first and then QuickTime.  That way Audacity.
> will behave the same on Windows, Linux, and OSX... this would mean a change 
> in behavior and the possibility that FFmpeg would take responsibility for 
> importing a file when the QuickTime importer may be a better selection.
> However, with the extended import/export option in preferences, the user
> would be able to override the order if desired, so there's no loss of 
> functionality.
To answer that, can you explain more about in what circumstances FFmpeg is
tried first and what extensions are involved - is it aac, m4*, mov? And I think
OpenFile has a "QuickTime files" or similar filter, correct? 

The way I see it "should" work is that the current setting of the filter in
OpenFile determines what importer is tried first, if there are no "extended
import" rules set, or if there are rules but we don't force them to override
the filter. However how it works now if you have no rules set is that the
"overrides" checkbox in Extended Import Prefs affects matters. The checkbox is
unchecked by default (OpenFile doesn't override), so when you go to File >
Open, choose the FFmpeg filter and import a WAV, you actually still use
libsndfile to import; to force that filter to use FFmpeg, you have to go into
Prefs and check the box although its wording "any rules" implies it should be
irrelevant if there aren't any rules. So question: should that box be greyed
when there are no rules? And if not, shouldn't the box be checked by default?
Otherwise, we're contradicting the Manual that says the OpenFile filter
forces use of FFmpeg:    
http://manual.audacityteam.org/index.php?title=File_Menu#Open... 

Basically the Manual was correct until LRN changed things so that any import
method respected the filter choice or the rules set. 

Now let's look at the scenario where we import a WAV with a rule set to use
libav for WAV. If the check box is checked (OpenFile overrides) and you choose
the WAV filter in OpenFile, the WAV imports via libsndfile. OK. Now uncheck the
box (OpenFile doesn't override) and choose the WAV filter in import files - the
WAV imports via libav. OK. And when you then use Recent Files or drag you also
get libav used. I think it's 100% correct the same importer is used
irrespective of open method, but some people read that checkbox as worded to
mean that "no-override" doesn't apply if you don't use the OpenFile dialog.

So second question: can we reverse the logic of the checkbox so it becomes
"Force importer order irrespective of filter in OpenFile dialog"? Or short form
"Rules override filter chosen in OpenFile dialog"? I think that would remove
the confusions people are having here.  

----

On the CLI/GUI speed craziness, clearly these two are the problems: 

* Windows (external program) export is unreasonably slow, any encoder. Start a
new bug? 

* Ubuntu "M4A files" export is unreasonably slow - but not (external program).
This is the major reason for the "slowness" part of bug 317. Can it be
addressed?