You are correct, two moods can’t be played simultaneously. I see your problem here, but as versatile as Syrinscape is, i am not sure if this problem could be solved technically.
Let’s say for a minute, that you could play two moods at once - this still wouldn’t solve the problem, that you would have to be switching from one soundset to another to trigger different moods. Given a fantasy scenario, you could easily break down the means by which your characters travel into the three you mentioned. So i wonder, what 12 moods you have. Different environments are definitely more possible, as are weather related moods. The resulting permutations would be far lower nonetheless.
Anyway, i am totally with you that, as a DM, your focus should be on the players and not being a DJ. With Syrinscape you have the tool, that doesn’t unbind you from a little preparation though. Maybe try it like this - you probably have a rough plan on how the PCs will travel. Prepare the different environments as moods (like desert, rain forest, plains…) - if you want to keep it real, in Fantasy, the average group doesn’t cross lots of different climate zones during one session - so you can cut down there. Plus they won’t have every means of transportation available, so you can cut down even more. So - let’s say you have three environments and three means of travel. This equals nine different moods which can be easily kept in one soundset. As they travel, the most likely thing to change is weather, so either: go to a weather soundset, where you keep all the weather you want and start elements there, or include those elements in the above mentioned travel soundset. So you have different means of travel and every weather you need in one set.
Maybe give it a try and see if this could work for you.