Many thanks for the feedback!
You are not the first complaining about the sound. It works quite fine in the installed version on my desktop with headphones. But on other setups the levels seem to be wrong. Some have only very silent voiceover, others have no ambient sounds (like you). I will try to fix that for the next update.
I will also improve the selection visual.
When it comes to defensive structures/units I am a bit unsure. Currently 4 additional units are planned. When those are finished I will do the multiplayer balancing ... and maybe come back to this idea.
I played YesterSol about a year ago. Will have a look again... :)