Hi, I noticed the same thing when I start Requiem via the 3DFX.exe. Then music does not play for me. When I start the D3D.exe, music plays. However, here you can not set the resolution higher.
The solution is to delete the D3D.exe or to make a backup in the folder of Requiem, for example D3D.BCK. Then make a copy of the 3DFX.exe and rename the copy of 3DFX.exe to D3D.exe.Then Requiem starts with 3DFX and music plays as well. But you have to pay attention when using the Windows start menu to start Requiem using the D3D shortcut. If you start from GOG Galaxy, click on "MORE" in the Rquiem window and on the bottom of the menu on "Other" and choose there D3D.
I hope, I could help you.