Finally I have some time to continue with this investigation and great improvements were achieved with the circuit provided by Ant: http://www.rev-ed.co.uk/docs/picaxe_sound.pdf
First here are the photos deleted by mistake from flickr:
Electrec thingey inside.
This is a closeup of the circuit, I´ve added a 10uF capacitor to stabilize the output signal (it´s the green cap on the right).
The problem here is that I can´t imagine myself soldering all this components onto a pcb, I would like to have at least two of these, four would be awesome.
Click the following button to see source code for Arduino and Processing