How to record a speaker?
AC couple the connection by using a series capacitor.
A 10 uF electrolytic (or non polarised if you had one to hand) would be an OK start but higher and lower may work as well (0.1 uF - 100 uF range may be OK). 

If, as is likely, the level is far too high for the mic input use a resistive divider .
A 10k or maybe 1k pot will make life easier.
A "pot" has 2 end of track terminals and a wiper terminal.
Connect as per diagram below.  

Start with the wiper at the common or "ground" end.
It actually matter too much (usually) which end you consider ground in each case BUT on the camera side the ground connection will usually be hard connected to the camera metalwork. Not always.   

Adjust pot slowly while noticing results.  

IF IT PROVES necessary we may then start in on impedance matching :-).
If.
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  I don't understand how I'm going to prevent the signal from the speaker breaking the microphone. 


Update: I now see what you mean by "... breaking the microphone."
The altered circuit may work as is. Failing that a very simple opamp buffer will allow one way signal feed.

The capacitor I suggested removes any possible DC component.
This may not be present but the cap does no harm.
The 1K (say) potentiometer reduces the level from about zero at one extreme to full signal at the other.
The combination will work well enough in many (most) cases.







simulate this circuit &ndash; Schematic created using CircuitLab







simulate this circuit



Here is a 4am, must go back to sleep rather rough basic mixer.
There are far far better on web and a search under "mixer" in images will produce examples to ask us about.
The LM324 is a quad op-amp that sells for maybe $1 (or 3 cents in volume in China as an indication of its ubiquity). 

IC1a and OA2 (whoops) provide solidish buffering against speaker to mic feedback - which OA3 also provides as OA3- is a "virtual ground". 
I may improve or delete this circuit later.

A few zillion examples here

Even something as simple as this, with a few pots thrown in, may well be good enough: