Well, you can't do it that way, but you can delete every mixfile your'e able to find dorectly on the cd from the game folder. The game will then read it from the cd. Also i think, you can extract and delete some MIXes (except for local.mix) from the ra2 and ra2md.mix with XCC Mixer and burn them to a CD, or probably the ra2- and ra2md.mix'es themselves, the game may find and use them. I know the game is searching for some things on the CD, when it's unable to locate them in game folder, but without warranty, I didn't test it.