Broken spokes are the result of undertensioned spokes… 
[url]Another broken spoke on back wheel]
[url]My Strida SD (Sport Duo)]
A few days ago I noticed a really ugly, creakly rhythmic noise, coming from the rear wheel of our silver 18" Strida (the wheels of this bike are for, maybe ~150 km in use).
Immediate checkup showed ~ 50 % loose spokes at the outer side of the rear wheel (next to the magnet) and ~ 30 % at the inner side (next to the brake disc).
Some of the spokes needed 3 or 4 full turns to tighten them…
The frontwheel seems better, there are loose spokes, but far less than rear.
(But the frontwheel has a radial runout of, at least, 2 mm 
)
Trueing info:
http://www.sheldonbrown.com/wheelbuild.html#tensioning
Now I’ve ordered a spoke tension meter
and am checking material for a trueing stand - more to come… 