to the OP, it sounds like the software you are using to "boost" the volume of your files has some sort of software EQ in it that stays resident in the phone memory until you power cycle the phone.
One of the other posters suggested numbing down the audio quality of the audio files on the phone to a sample rate of 8Khz (BTW...it is spelled kilohertz

). For most males this would make the files louder since at the 8k sample rate, most of the low end and high end of the audio would no longer be present and the audio frequencies that are left are in the range that most males preceive as loudness.
One thing that you can try is to edit all of your files with a program like CoolEdit 2000 (google search it) and when in the editor, you can "normalize" the audio which will increase the loudness of all the frequencies of the file. Then load the fixed file to the phone.
Otherwise, you could try to find EQ software for the phone. I have not seen a standalone software product for WM6 like that, but then again I haven't had a need for it. Actually, thinking about it now...the Q9h does have an audio enhancement section of the settings area where you can manipulate spacial audio and bass boost. It can be found in Settings>More...>Audio Enhancement
Good luck!