Mostly we'd need generic background music for each stage of the game, each one giving a different feel. If you're unfamiliar with them, they are: microbe, multicellular, aware, awakening, society, industrial and space. We've completed the microbe soundtrack already, but you could still compose something for it if you want and if we get a couple more we'll update the list.
There are also numerous scenarios that could occur in-game in one of those stages. For example, in the industrial stage, different national policies or government types could have different themes (communism, democracy, etc.), and there could be combat and peace themes for each.
If you want a specific suggestion...hmm, we don't yet have anything for the awakening stage or much for the society. These are the stages where a species stops evolving and begins developing the first primitive technologies, up to pre-industrial.