I can remember that Dark Reign had a very good Soundtrack. Will it be available as bonus material?

Nope it doesn't look like it.

Just look on youtube.

the soundtrack comes with the game in it's root directory

Here are a couple default examples where you can find the soundtrack depending on what operating system you are using.

32 bit
"C:\Program Files\GOG.com\Dark Reign\Sounds"

64 bit
"C:\Program Files (x86)\GOG.com\Dark Reign\Sounds"

I was just about to give my physical copy of the game to a charity shop (have a copy on GOG and wanted to save space) then just noticed that the CD has the soundtrack as CD audio tracks. Just ripped it to FLAC for preservation and safe keeping. Shame GOG doesn't do lossless audio for folks like me.

I noticed the folder only includes the music from the original Dark Reign release though, the expansion Rise of the Shadow Hand had a new, and in my opinion even better soundtrack. Here's the theme song from it: https://www.youtube.com/watch?v=TQa4PtdWCow