and thanx Paul-B, as well, for the answer 
i tried at first to sink the 3.3V coming from the MP3 board directly through the pin, with my "welded" cable 
hooked to D7 (configured as OUTPUT with LOW pulses, blink-style) via a voltage divider (10k--20k). it didnt worked. any idea why?
and thanx also for the english lesson. in spanish we dont make the difference between soldering and welding, and i get it wrong all the time hahaha... plus: i work as a welder...