Thanks to everyone for their suggestions!
It turns out the weakest link in the audio system I'll be using for
the presentations is the soundcard in my old Gateway laptop. (Not too
surprising since I reported problems when it was new and Gateway tech
support said, roughly, "Too bad" and made no offer to help.) The card
makes a constant high-pitched tone in speakers that can be heard if
the signal is amplified but can't be detected through headphones. And
the level and nature of the noise increases/changes when there's lots
of hard disk activity. I observed the same problems with two different
amps/connection wires and two different sets of speakers, so I really
think the problem is in the computer.
Still, I think I can adjust amplifier levels and the playback levels
in the PC to get by without many people noticing any noise.
Let me outline some of the important points that people suggested and
that I've had time do so something about --- perhaps this will save
reading through lots of posts if anyone else is interested:
1) Make sure that the sound files are all normalized to the same level
so that playback is at roughly the same volume w/o needing to adjust
levels during the talk.
2) Listen before the talk, ideally after some audience is seated, to
make sure sound levels are reasonable. Get them to sit where the sound
is best.
3) Get speakers up and away from walls and corners, pointed into the
center of the audience from either side
4) Make sure you can get high enough playback levels for the room
you'll be in.
I'm guessing that many people won't even notice the difference between
the stereo and mono tracks I'll be playing. Indeed, I hope they don't
because what I really want them to be paying more attention to what
I'm trying to tell them.
Thanks again to everyone for the advice --- I'm sure the presentation
will be better (and I'll be more confident about it) for your kind
attention to my query.
Steve P
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