I'm using an Arduino ATmega 644. I hope you guys can help and suggest! Been looking a lot in the net and although I found some it doesn't work.
As I know you're doing a sax, what I'd suggest is using the ATmega644 for the blow and key inputs. Then, look for an audio module that does high quality sampled audio output. The ATmega takes care of all the inputs, then triggers a specialized audio output module to produce the sound at a much higher quality than the ATmega could ever produce.
While the ATmega is good at many things like monitoring many inputs, audio output is not one of them. At the same time, there are other chips that are excellent at producing audio, but are not good at all at monitoring many inputs. Using two systems that talk to each other is the way to go in this case.