Microburst, there are indeed waves not associated with mountains. We sometime get them in East Anglia, UK, not noted for its sticking up bits. In my experience, and from what I have read, they do not generate such strong up and down velocity vectors as mountain waves, but for all I know there may be exceptions to that. Wave has also been seen on satellite photographs over the sea, far from any mountains – though with favourable conditions, mountains can generate waves a long way downwind as well as close to the triggering source.
One mechanism for wave without mountains AIUI is wind shear. Think lower high density fluid, with higher low density fluid having relative motion to it – such as sea/air interface. It seems that can happen higher in the atmosphere too, with adjacent layers.
(Glider pilot, not met man, and not ATPL)