I feel like the implication that a bard can cast spells while playing music means he can preform the necessary actions while playing music. Why would he play a lute or whatever to cure his party or something but then put it down and wave his hands around? I don't see any reason to assume that the somatic components for each class are the same thing. For the bard wouldn't his somatic component to his spell casting, which is music in his case, be the somatic component of his music? So for a bard who is playing a harp, yes his somatic component would require a free hand, or in that case more likely HANDS. But for a bard who is singing his somatic component would really be in the capacity to song.

Even for classes who share spells, I don't see why the way in which they cast them has to be identical.