Check your Freelancer folder, there should be a folder for the music pieces playing in the background. You can replace those with the music of your choice. Take note that they have to be in the same file-type as the original tracks, WAV I believe. Also, make sure you replace the original files, meaning your files should have the same name, for example: new_london_awesome_track or something, whatever the names in the folders are.
I can't find the topic right now, but I know there is a sound pack mod for changing the noises of the guns to a better sounding note, I used to have it. Just can't remember the name of the modder.
Then I think the same person created a Final Fantasy pack where the music was done to the the final fantasy games, like Fight on! during Liberty battle soundtrack I think. Basically it was a music replacement pack.
Dii did that, or Hidamari, his forum name. Still got that mod lying around here, just search for it on the forums, if you dig the older Final Fantasy games, amazing.