Hard surfaces in your conference rooms, such as the large glass walls and windows mentioned above, along with large drywall surfaces, large marker board surfaces and bare walls and floors will reflect sound waves for a longer period of time than soft surfaces like cloth-covered chairs and carpeted floors.
The prolonged sound wave reflection is known as reverberation. Too much creates a buildup of sound that makes a room louder and covers up the direct sound, such as the voice of the presenter.
Some simple solutions including swapping out hard chairs for cloth-covered ones, installing carpet or hanging curtains. More technical and robust solutions include acoustical ceiling tiles, ceiling mounted baffles and wall-mounted acoustical panels.