Try extracting the music from "C:\Program Files\EA GAMES\The Sims 2 Nightlife\TSData\Res\Sound\Splash.package" (assuming default NL install directory), and replacing the file in "C:\Program Files\EA GAMES\The Sims 2 Open For Business\TSData\Res\Sound\Splash.package" with the extracted NL startup music.
I've forgotten the exact process for doing this though, but that's a rough outline. I'll try getting some pictures later.
EDIT: It's as easy as extract, save, replace. Remember to back up the OFB file before trying this!
ExtractImageOpen "C:\Program Files\EA GAMES\The Sims 2 Nightlife\TSData\Res\Sound\Splash.package", right click on the mp3/xa file, and hit extract.
SaveImageSave the file to a location where you know that you will find it. I've chosen the desktop, but it can be anywhere.
ReplaceImageOpen "C:\Program Files\EA GAMES\The Sims 2 Open For Business\TSData\Res\Sound\Splash.package", right click on the file, hit replace, and find the file you saved. Job done.
IMPORTANT NOTICE: This has
not been tested in game, as Open For Business isn't running properly for me.