Basic idea:
Copy over the Diablo II Expansion Movies and Diablo II Expansion Music files from the Lord of Destruction:Support folder into your Diablo II Files folder.
Extra Spam:
Do you guys remember how the Diablo II Standard doesn't install the Diablo II Music file by default? Well, the Diablo II Expansion pack installer does, but only because it has it's own Music file on the LoD playdisc. I noticed in Act 5, there were long pauses while I had Music on. I killed the Act 5 Quest 1 Boss and there was a long pause and music played while I had music turned off... Well, after a 1.5 hour solo fight with Diablo (my Assassin has some pretty bad equipment), the game played some music for defeating Diablo... but the music started after a 5 second pause while the game waited for my CD to spin up... I looked on the Play disk in the Support folder and there were 2 files that weren't installed...
Diablo II Expansion Movies and Diablo II Expansion Music. Install those into your Diablo II Files folder and you won't get those annoying CD spin up pauses....
Well, if you turn off music you don't need to copy it over, you say? There are cases where it automatically plays victory music at certain times as witnessed when I defeated certain bosses.