AIUI there's a PA /gstreamer plugin that does the mp3 decoding in DSP for mafw. madiaplayer just delegates the whole mp3 file to mafw to play it. I dunno if this special plugin directly routes to DA-converter aka soundcard, or this is done the standard way of gstreamer plugin chains where this DSP plugin would stream the plain PCM (aka "wav") audio data to next plugin. IF the latter, then you of course could change the properties of (or the whole) sink plugin.