It is not a problem with Maemo Mapper or Flite but rather a well known issue with OS2008HE (the speaker volume is very low, supposedly because it is expecting stereo speakers and there is only one). In my case, installing ogg-support seemed to increase the speaker volume for some applications (Maemo SID comes to mind), but it does not seem to make a change for flite.

I would be interested to know if you find a workaround for flite...