So I  explain my question as:
  if I use x-lite softphone in speex codec(it must be narrowband), the vioce quality is pool .
 but when I use X-lite in speex wideband codec , the voice quality is very nice.
I do try to change the parameter of default_codec_settings in mod_speex.c , but it seen don't

affect anything.
  so who can tell me how to ajust the freeswitch side to fit the X-lite softphone in speex narrowband?

best regards .
your freeswitch' fan.