I've been lucky enough to obtain the the Halo Wars: Original Soundtrack, though not particularly early or anything. After giving a good listen to Ensemble's first Halo project, I must say I'm impressed.
The soundtrack was composed by Stephen Rippy and hit shelves the 17th of this month. In the Halo: Combat Evolved soundtrack, there was an almost equal dichotomy between emotionally stirring epics and low, but intense synth sounds. Games after the first, particularly Halo 2 and Halo 3 chose the former, heavily orchestrated pieces with very light interludes of synth. Halo Wars goes in the opposite direction. While there are a few really nicely done orchestra-style pieces, most seem to fall under the old synth sounds, calling to mind a cross between Metroid and Halo. I find this decision to stick out well amongst Marty's works, and it certainly has a lot of nice bits. It lacks the instant "hummability" of a lot of Halo tracks, but trades it for a unique, alien sound that hasn't been a part of the Halo musical repertoire for some time now. The few pieces that do follow tradition, such as Spirit of Fire are not only emotionally stirring, but a fitting introduction to a war that often is painted as epic, but in reality is quite tragic.
My main gripe with the soundtrack is the complete lack of a total remix; though this will seem as a plus to some, I wish there was a track to pull me back to the previous games, to audibly connect me to the past of the games, and the future of the timeline. I fully understand Rippy's choice to let Halo Wars stand on its own, however, and sometimes it feels stronger for it.
Overall, I'd say the Halo Wars: Original Soundtrack is a nice experiment in the other direction, using alien beats moreover to describe the theme of environments than to describe the themes of battles, as Halo soundtracks tend to do traditionally. It is somewhat flawed, but so were the experimental bits of the Halo: Combat Evolved soundtrack, and that remains my favorite of the series. Its weaknesses are its strengths, and vice versa.