Hello.
I have been searching the internet for 2 Days, and i didn't find anything.
So what i want to do, i own the Arduino MEGA and UNO and the ethernet shield.
Now, is it possible to play midi files, that are stored on the SD over a speaker? Without any additional Hardware?
Sure, as long as you don't mind square waves and limited polyphony. 
Of course you can add a shield to get full MIDI instrument: http://shop.moderndevice.com/products/fluxamasynth
Thanks, that looks very good, maybe i'll buy it
So, you said i can do it without the shield, but limited.. How?
Science:
Thanks, that looks very good, maybe i'll buy it
So, you said i can do it without the shield, but limited.. How?
There is a third-party Tone library that can play two simultaneous square-wave notes on different pins:
http://code.google.com/p/rogue-code/wiki/ToneLibraryDocumentation
You could generate square waves in software to get multi-part harmony. We did that back in the Intel 8080 days (1975 or so) to do four-part harmony. I expect the ATmega would be at least as capable. Every clock tick you add a constant to an integer and if there is an overflow you invert an output pin. A constant of 0 gives you silence. A high value gives you a high pitch (1000 give you about clock/65.536) and a low value give you a low pitch (65 give you about clock / 1000). Minimum pitch is 1 (clock * 1 / 65536) and maximum is 65535 (clock * 65535/65536). You could use unsigned longs to get even more resolution.
The Arduino can get 8 or more note polyphony, with different waveforms and volume control for each note. So not too limited, it's more than an NES or gameboy!
Check out my project, and joemarshall's
My synth can already take MIDI input, it shouldn't be hard to adapt it to work from MIDI files.