The macro icons are pulled from icon_ms1.dds
If you edit the images in there, they will be used instead of normal (Note: replace the file in your custom UI folder, not default, or the launcher will replace it)
Also note that other things such as "Follow" or "Yell for Help" use some of the icons, so be careful what you swap, or you'll end up with those looking like spells
Edit: as for music, make a "custom_music" folder in the eq2 folder, then a "Zones" in there - drop some mp3s in that are named like the ones in Music, and voila, instant music replacement