[crunch]

I don't know about animated load screens. As far as I know the game only supports .tga images for loaders.
Music, on the other hand, I can help you with. I have been trying to get it to play right out of the .pk3, but can't get it to work that way. But--- if you place your .mp3 file into your main>sound>amb stereo folder, and then point to it in your .mus file, the .mp3 will play in your level. (You can use triggers to have different .mp3 files playing in different areas.)

As far as music during the loading process, I don't know. Not even the original boards have loading music. The level soundtracks start after the load is complete. Whether this can be scripted to run with a load screen... that may be possible, but I don't think so. Judging from console output, the loading screen shows up and then the scriptmaster loads the script.