I've noticed the MP3 player volume low too.
But only with some songs.
Maybe get yourself something like the Android 'PowerAMP' App.
PowerAMP – Music Player for Android
It has a graphic equalizer and I got a bit more volume out of it.
But didn't test it 100% last night as I was tired and slept.
I will give another update on all of this
when I study more over the next couple of days.
I also have Cool Edit Pro,
and AVS Video and MP3 Converter.
So, if I had an MP3 documentary or language lesson or songs that I loved,
I would convert them to WAV, then up the volume in Cool Edit Pro,
then convert back to MP3, and USB it to my Android phone.
Android Forums - John Android's Albums