Glass is a pretty good conductor of heat and cold, so you put a cold drink on room temperature glass and condensation happens, which can happen on the bottom of the glass too. Meaning you could be getting moisture into the veneered top of your speaker. Another problem with glass directly on top of finished wood is the finish can't breath. There should always be an air gap between the wood and glass. Even then, if the glass doesn't cover the entire wood surface under it the exposed wood will oxidize differently, so it'll end up being a different color tone one day down the road.
The same applies to stone, granite, marble, etc.
Personally, I would never put anything on top of a speaker. It's just asking for trouble.