source amplification question for my new setup

[rain]

Hi group,

I just setup a new home system w/ a harmon kardon AVR 55 and four 3-way
homebuilt speakers. A CD or DVD stereo source replicated over the
surround sound is great, however, a computer mp3 source is lacking in
volume in comparison. What can I do to rectify this difference,
considering that I listen to mp3's exlusivly. (flame here!)

Thanks a bunch in advance. I really appreciate it.
Brian