Thanks for the tips. I've done some more experiments, mayby someone can
(re-) use the results.

My aim is to produce a few different sounds (at least one for shooting, one
for being hit and one for being killed) which are both loud enough to be
heard from some distance and appropriate for their function. I've finished
the electronics, now I'll start exprimenting with waveforms (using PWM).

ELECTRONICS

The most simple solution is to couple a PIC output directly to a speaker,
with or without a C. This gives a verly low volume due to the limited drive
capability of the PIC.
                      ___
                      \ /
    PIC --------------| |----- Gnd
                   Lsp -
                      ___
              ||      \ /
    PIC ------||------| |----- Gnd
            C ||   Lsp -

Direct PIC output with and without capacitive coupling: simple, but not much
volume due to PIC impedance.

The one-transistor solution has the drawback that current always flows
through the speaker in the same direction (switched DC), which in the long
run could be bad for the speaker, and limits the movement of the conus: it
can only move from the middle position to one of the extreme positions. And
when you leave the pin in the wrong (is this case: high) position for too
long quite a current will flow through the speaker, not a good idea for a
battery-powered application.
                               ___
                               \ /
                          /----| |---- Vcc
            R  ____     |/      -
    PIC ------|____|----|     Lsp
                        |>
                    npn   \
                           Gnd
 or
                          +------- Vcc
                        |/
    PIC ----------------|
                        |>     ___ Lsp
                    npn   \    \ /
                           +---| |--- Gnd
                                -
single npn buffer: more volume, but not optimal (DC problem)

That's why I initially choose a npn/pnp pair, which gives a real AC of Vcc
Top-Top (effective voltage is lower: 0.5 * Vcc for a square wave,
1/2*sqrt(2) * Vcc for a sine).

               npn    / Vcc
                    |/
                +---|            ___
                |   |>\    ||    \ /
   PIC ---------+      +---||----| |--- Vcc or Gnd
                |   |</    ||     -
                +---|        C     Lsp
                    |\
               pnp    \ Gnd

complementary buffer: good volume, real AC, low impedance. Can not make use
of a Vcc > PIC-Vcc

In my application I'll have a 7-9V NiCad which is regulated down to 5V for
the PIC and the SFH506 IR receiver, so I can gain some volume by using the
NiCad directly. This requires a level shifter (3th transistor).
                         ____
                    +---|____|------+---- Vcc > Vcc-PIC
                    |             |/
                    +----------+--|           ___
          ____    |/           |  |>\    ||   \ /
   PIC --|____|---|            |     +---||---| |-+
                  |>\          |  |</    ||    -  |
               npn   \         +--|               |
                      Gnd         |\         Vcc or Gnd
                                    \ Gnd

complementary buffer with inverting level shifter to make use of a higher
Vcc than the PIC's Vcc.

Once the impedance of the driver is no longer a problem the volume is
determined by the impendance of the speaker and the voltage applied to it (
P = V*V/R ). So to get a high volume it is important to take the lowest
impedance speaker which you can get (4 ohms if possible). It is even more
important to apply the highest possible voltage. If you only have 5V it will
help to use two PIC output pins which are always each others inverse to
drive two complementary buffers. If you do not want to use two PIC pins an
extra transistor can provide the inversion. And if you want to use a Vcc >
PIC-Vcc you'll have to add two level-shifters (no need for two PIC outputs
then, the level shifters are also inverters).


               npn    / Vcc
                    |/
                +---|      ___
                |   |>\    \ /
   PIC1 --------+      +---| |-+
                |   |</     -  |
                +---|          |
                    |\         |
               pnp    \ Gnd    |
                               |
               npn    / Vcc    |
                    |/         |
                +---|          |
                |   |>\    ||  |
   PIC2 --------+      +---||--+
 (inverse       |   |</    ||
  of PIC1)      +---|
                    |\
               pnp    \ Gnd

dual complementary buffer (bridge) doubles the voltage compared to a single
bridge.

MECHANICS

One non-electric measure which is quite important: provide a good housing
for the speaker. A speaker without any housing is just causing local air
movement (from the front of the conus to the back and v-v). Ideally a
speaker should be mounted in an infinite, non-flexible plate. If you don't
have that a box will do.