It was basically Borealis music at night. After a few years, PC upgrades and ISP improvements made it impossible to hear before AO loaded.
The music files reside in the main data directory for the game itself - with most in a readable (.wav file) format. (Funcom\Anarchy Online\cd_image\sound). The music directory (music\env ) would be my best guess to start.
I also think there was a CD made at one time with all the ambient music for an anniversary or promotional of some sort.
I found my copy of the CD, it has 14 tracks on it, I also found a link to this collection on Spotify which seems to have a few different ones:
But it also seems to be a dead playlist.
Thanks much for the responses.
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I finally found it: https://www.youtube.com/watch?v=9bP8LQkQ1mY
Thanks to YouTube Music making an automatic playlist of music videos I like. I haven’t been really happy with it compared to Google Play Music, but I’m beginning to make peace with it now.
I’m pretty much bored out of mind with new games. I guess it’s time to bite the bullet and play AO again - I upgraded my PC and forgot to preserve the character ini files. 
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Heya.
Happens that i’ve contributed to gather 4 volumes of AO music in 2017 (56 tracks in total).
The one you specifically requested is from volume 1 & part of track “Serene Mountain” (n° 12 in 2017’s final edition, but n° 11 in old RK music CD Funcom released in 2001) by Morten Sørlie.
This nostalgical loadscreen theme starts around 3:30 for more accurate timing.
You can jump right to it in this YT repost https://www.youtube.com/watch?v=A9EClbtSr5E&t=53m45s
Feel free to also check other volumes : n° 2 is an extended version of old SL music CD Funcom released in 2004 ; n° 3 (focused on AI/NW) & 4 (centered around LE/Scifi) are totally new playlists i’ve had the immense pleasure to conceive with the great help of all composers & musicians that contributed AO’s music.
Can’t refrain to direct you to Steam’s AO page as it’s a free pack to obtain all quality MP3s. Another good way is this FC ftp server Directory index - styled with h5ai
Enjoy 
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