1) When praying, one should not stand on a high place (e.g. on a chair, stool or bed), as he will be preoccupied and worried that he will fall, and will then be unable to concentrate on his prayers. Furthermore, it is incorrect to stand on a high place during prayers, as there is no haughtiness before Hashem. If he is old or ill and is unable to pray ion any other way, it is permitted. (Shulchan Aruch and Mishne Berurah 90)

2) If the reason for standing in a high place is to allow the congregation to hear his voice, it is permitted. However, some say that this clause only applies according to the reason that “there is no haughtiness before Hashem”, but according to the reason that he might be preoccupied and not concentrate because he is worried that he will fall, it is still forbidden. (ibid)