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thanks for the replies. here's a long winded post mortem....
although i'm not completely convinced that it is solved, i feel pretty
good that i've fixed one part of the problem. the pointers about
feedback from the microphone led me to further investigations and
after asking the right questions i found some man pages that happened
to be the right utilities. it hadn't crossed my mind that the
microphone would be even active---i guess that i just figured that
linux ignored it (well, maybe it does---i it could be the bios that
set it up).
so: for those interested, linux does have controls for the mixer.
they're "aumix" (command line and curses) and "xmixer" (X11). it
turns out that the mixer was set for 7%. this doesn't sound like much
but at a volume level of 3.5 or so tapping or brushing a finger over
the microphone causes audible scratching. even though the microphone
and speaker are on opposite sides, closing the case must create a
sufficient channel that the feedback becomes significant under the
right circumstances. by turning the microphone mixing percent up
(testing!) there was strong and clear feedback. so i set up an entry
in the rc.local script to turn this off during boot. i'm counting on
this happening early enough not to be a big deal but decided to punt
using the SVR4 init levels and just add the feature to the bsd-like
rc.local that is built in to the SVR4 init tree by red hat for lazy
bums like me. if there are other bums out there who might want this
script, here it is in all of it's simplicity (substitute the
appropriate path if required!):
# fix the audio so that we don't hear feedback from the microphone
if [ -x /usr/bin/aumix ] ; then
echo "audio: microphone is OFF to prevent feedback";
/usr/bin/aumix -m 0
fi
and a final note---it seems that aumix's curses mode screws up the mouse
buttons in the xterm that it is run in so beware!
so thanks for the replies---they definitely helped!
---K