If boosting the volume using software surgery on the music file itself hadn't helped, you could go for a hardware solution, like one of these:
http://www.amazon.com/Fiio-E3-BK-E3-.../dp/B001MPWMDA
http://www.amazon.com/Upbeat-Audio-T.../dp/B000EIWCFE
Or make your own:
http://tangentsoft.net/audio/cmoy-tutorial/