How does this telephone circuit modulate the current?
There is a pretty high DC voltage (48 V) between both wires (called
  tip and ring). I still ... don't know which one is connected to ground, if tip or
  ring.


The ring is about -48 volts with respect to earth ground (just remember the phone system emergency power is the equivalent to 4 car batteries and you will easily remember this voltage).  The tip is about zero volts with respect to earth ground.  By the way (BTW), this is the on-hook voltage.  As soon as you go off-hook the voltage drops as tip and ring now carry sounds (dial tone).


  The voices are modulated on the AC current, not on the voltage...


If you have voltage and a load you have current.  They are all related:

V  = I x R


...so both current and voltage change when a voice is being sent over tip and ring.


  At audio frequencies, 


Phone audio is sampled at 8K bytes per second so the upper frequency cut-off is 4KHz.  Nothing higher gets through a normal phone connection.


  the impedance of the speaker should be fairly
  resistive ...


As said before, if you measured both the voltage between tip and ring and the current through tip and ring you will find both are changing according to the audio being sent over the tip and ring.


  You get feedback on the speaker, right? The microphone signal goes
  through the transformer...
  so that the outgoing signal doesn't overpower the
  incoming signal...


A POTS (Plan Old Telephone Service) line is a 2 wire interface. But it essentially carries a full duplex conversation.  To accomplish this the local hand set needs to remove the local sound from the local ear piece.  This is done using a cleverly wound transformer which subtracts the local sound before sending the signal to the local ear piece.


  A2 is the microphone, represented as a variable resistance, which I
  don't understand...


The off-hook voltage is about 3 to 9 volts.  This is supplied by the phone company.  Old phone microphones were made using carbon and would change resistance when air pressure waves hit the diaphragm.  Again, as the resistance changes so does the voltage and current as given by the equation:

V = I x R



  ...most modern phones use electric microphones,
  which just generate a small voltage proportional to the sound
  pressure. How is this implemented on a modern phone? ...


No matter how a modern phone is designed, it always need to emulate the old POTS standard.  That is, all phones must appear the same to the phone company's equipment.  So it does not matter how the microphone works in a modern phone.  As long as the audio modulates the voltage and current over the tip and ring the same as the old POTS standard.


  What are the typical amplitudes of the AC currents?


Assuming you are referring to the ringer signal, it is about 90 volts at 20Hz.