I know this is kind of late, but wouldn't it make more sense to have this sound in player.xml?

Maybe, but then you have to query the player? Also one might want different invalid placement sounds depending on the unit, given their choice of buildings might be different.

g_viewedPlayer? Okay. I thought more of a Red Alert-like constructing, where the sound still needs to be played and there is, not necessarily, a unit selected.