first thank you for your replies.
i tried both a ported version and the original verilog i2c controller (as your audio_codec_controller requests ans i2c_controller component).
during lunch break i made a new project and i was not able to reproduce "varying static". but static is still present when sw1&9 are up (adc dac loopback), as moou pointed out, with or without input signal on line in - i've tried different i/o audio sources and the problem persists.
initialization works fine as expected (5 on hex0) and sine/square wave generation works well too.
maybe i'm missing some macroscopic point. anyways thank you for your help.