Hi list,
Once again, I did some basic learning on my own, and was able to come up 
with a passable solution, TIMTOWTDI. Probably far from idiomatic, but it 
gets the job done in my basic testing.

I looked at the flymake code, finding the flymake-report-status function 
called with the error and warnings count string, formatted like "%d/%d". 
Looking at the calls to that function, the only one having a non-empty error 
string is the one that posts the updated error and warnings counts on the 
mode line. In case of no errors or no warnings, it does not post 0/0.

Here's what I added in my .emacs file:

(defadvice
   flymake-report-status
   (before beep-on-flymake-failure (e-w &optional status) activate)

  "Play not_ok.wav in ~/emacs if we have a non-empty error count string."
   (if (and e-w (>  (length e-w) 0 ) )
      (play-sound-file "~/emacs/not_ok.wav")
   ) ; if
) ; advice

The ending comments make it easier to mentally track the parens, with a 
screen reader.

I have also realized that playing another wav in the else case is a bad idea 
as every succesful syntax check would beep reassuringly. That's far too 
often to be gratifying, <smile>. Plus the sounds play synchronously, on 
Win32 at least, ouch, so I'm keeping things brief. As in my OS sound scheme, 
the error is a 40 ms quiet burst of pinkish noise.

On a side note, Is there an easy way to only get the syntax chekc after a 
time out, on leaving the current line? OFten I leave the cursor on what I'm 
editing and start thinking, yet my statement may well be in a syntactically 
incorrect state at that point. I don't always hit C-j at the ends of lines, 
either, if I'm editing an existing statement in stead.

Hope this helps somebody else some day, too.

-- 
With kind regards Veli-Pekka Tätilä
Accessibility, Apps and Coding plus Synths and Music:
http://vtatila.kapsi.fi