Replace plasma sound effect with one generated using Csound.

This is the first sound effect which is generated from scratch
using Csound. The advantages of this are:

- Real source code for sound.
- Anyone with a text editor can tweak the sound effects.
- Source code is much more compact that the sound file itself.
- Sound files can be regenerated with arbitrary quality.

sound/Makefile

@@ -0,0 +1,16 @@
+SFX = plasma
+OGG = $(SFX:%=%.ogg)
+WAV = $(SFX:%=%.wav)
+
+all: $(OGG)
+
+%.ogg: %.wav
+	oggenc $<
+
+%.wav: %.csd
+	csound -g -o $@ $<
+
+clean:
+	rm -f $(OGG) $(WAV)
+
+.PHONY: all clean

sound/plasma.csd

@@ -0,0 +1,25 @@
+<CsoundSynthesizer>
+<CsInstruments>
+sr = 48000
+ksmps = 32
+nchnls = 1
+0dbfs = 1
+
+; Plasma shot:
+; Basically a sine wave sweep from 2 kHz to 0.1 kHz in 0.1 s,
+; but with white noise with an amplitude of 500 Hz applied to the frequency.
+
+instr 1
+	gisine ftgen 1, 0, 16384, 10, 1
+	k1 line 2500, p3, 100
+	k2 linrand 500
+	a1 poscil 0.2, k1 + k2, gisine
+	out a1
+endin
+</CsInstruments>
+<CsScore>
+i 1 0 0.1
+e
+</CsScore>
+</CsoundSynthesizer>
+