Hello Ian,

here is a quick and very dirty solution for your microphone trigger. I
found it in an old hardware book for the C64 and I have not tested ist
so far, but up to now there were no hoaks or fakes in this book.
The solution requires a CMOS 4069 wich is "mistreated" as an amplifier,
two resistors, a diode and a crystal(!) microphone. Trying ASCII-Art...

                     ---1.7----  1.7MegOhms, increase: -> more
sensitive!
                     |        |
         GND----MIC--+----D---+--|>|---+---D----D-----   to the PIC Port
                                       |
                                      1.5 MegaOhms
                                       |
                                      GND

Diode: 1N4148
D : 1/6 4069

If you have further questions about this nice :-) little drawing please
contact me. I can scan it for you.

I hope this helps
Markus

p.s. short sounds will cause short signals. Make shure your impulse
scanning with the PIC is fast enough!

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.