The announce sound was created using sox, the freely downloadable sound conversion tool which can also play almost any kind of sound file, but what few people know is that it can also create sounds and write them to a file.
The command I used to generate the announce sound is (all on one line):
play -n synth -j 3 sin %3 sin %-2 sin %-5 sin %-9 sin %-14 sin %-21 fade h .01 2 1.5 delay 1.3 1 .76 .54 .27 remix - fade h 0 2.7 2.5 norm -1
To record the sound to a file instead of playing it use 'sox' instead of 'play' and give the name of the file to be written to after the '-n' option:
sox -n announce.wav synth -j 3 sin %3 sin %-2 sin %-5 sin %-9 sin %-14 sin %-21 fade h .01 2 1.5 delay 1.3 1 .76 .54 .27 remix - fade h 0 2.7 2.5 norm -1
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Read the documentation that comes with sox to learn all the capabilities.
There are many cool sounds sox can create on-the-fly. Here are a few from simple to more complex:
play -n synth 0.2 sine 500-50
play -n synth 0.1 sine 500-2000 chorus 0.7 0.9 55 0.4 0.25 2 -t
play -n -c1 synth sin %-12 sin %-9 sin %-5 sin %-2 fade h 0.1 1 0.1
play "|sox -n -p synth 2" "|sox -n -p synth 2 tremolo 10" stat
play -n synth 2.5 sin 667 gain 1 bend .35,180,.25 .15,740,.53 0,-520,.3
play -n synth 0.5 sine 200-500 synth 0.5 sine fmod 700-100
The announce tones backwards
play -n synth -j 3 sin %-21 sin %-14 sin %-9 sin %-5 sin %-2 sin %3 fade h .01 2 1.5 delay 1.3 1 .76 .54 .27 remix - fade h 0 2.7 2.5 norm -1
An amazingly realistic guitar strum
play -n synth pl G2 pl B2 pl D3 pl G3 pl D4 pl G4 delay 0 .05 .1 .15 .2 .25 remix - fade 0 4 .1 norm -1
Realistic pluck of a string
play -n synth 4 pluck %-29
play -n synth 4 pluck A3
This last example sets up a loop using linux's bash commandline to play a sequence of notes (twice each) one after another. Other computer commandlines would use slightly different syntax.
for n in E2 A2 D3 G3 B3 E4; do play -n synth 4 pluck $n repeat 1; done