Concierge Hotel TV - Volume Control

Question

Hello,A few weeks ago I got two used 20" Concierge hotel tvs on ebay. I don't know the model cause there's no lable.The menu is disabled, so I cannot change the settings. In the firstdays of use the sound was OK. It could be turned upto level 65 on thedisplay (though it was muted from Level 63).One day it setted itself to another maximum volume. Now I cannot raise the volume above Level 45. I can hardly hear the sound. What can I do? Is it possible to unlock the volume control?