The negative lead is marked on the case and that is the shorter wire in most instances. 16V is the maximum working DC voltage that is allowed on the capacitor. You can use a capacitor with a higher voltage rating where a lower voltage is specified. In the circuit, C13 and C12 are used as reservoirs of charge. C13 is needed when an old style zinc-chloride battery is used rather than an alkaline type. Without C13, the high currents taken by the speaker make the battery voltage dip, and the circuit becomes unstable. C12 in conjunction with R16 stop similar supply rail variations from getting into the sensitive radio frequency amplifier stages. C10 allows audio signals to bypass R11 to keep the audio gain high in transistor Q4. C11 blocks the DC present on the audio amplifier output, while allowing the audio signal to pass to the speaker.
All standard electrolytic capacitors need to be connected the right way round.
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