If you are looking at the audio amplifier power rating on a TV the number is completely useless as there is no standard as to how the spec is measured and the amount of distortion at that power level. Even if the rating from one TV to the other could be compared how loud it can actually play is also dependent on the efficiency of the TV speakers. 10 watts to 20 watts is a 3db difference so if the TV speakers on the 10 watt are 4db more efficient than the ones on the 20 watt TV the 10 watt TV will play louder.
I would say that a more useful measure is the size of the speakers but overall built in TV sound sucks.