Firstly, thank you so much for your interest in the sound design aspect of LoL!
The answer to this question could be insanely long, so I'll try to distill it down a bit.
Much the same way that concept artists, animators, VFX artists, etc at Rito approach any new piece of content that goes to players, it all starts with a WHOLE lot of love. We're gamers and we try to think like gamers.
Sound designers are afforded the awesome opportunity to be in on the ground floor of any new Champ or skin, and it all starts with group brainstorming around trying to define who and what we're trying to create. From a sound perspective, we treat sound design very much as an iterative process, trying new things, recording new things, putting things in game, realizing what works, what doesn't and building on that. We play-test quite frequently, trying to understand what might get annoying in any phase of the game, and trying to optimize for that.
That always involves recording original content that we can slice and dice to use as pieces/parts for the sonic palette that will eventually define who and what a Champ or Skin is. Sometimes, those single recordings alone are enough to sell the fantasy of that character, but more often than not, it requires layering those recordings with other bits we have that might not have made the cut on an earlier Champ or skin.
For something like Bard, he was such an unknown entity in the the League universe, and as such, his sounds were primarily constructed out of new content created specifically for him. For Champions that come from factions that are more established, some of their sounds might evolve out of earlier recordings created for a previously released Champ from the same faction, but might not have worked out for their kit and share similar tonal qualities that lend themselves to the new Champ. Those sounds get re-worked and sculpted alongside new recordings that are unique to that Champ.
Does that shed some light on it? I'd be happy to try and elaborate on any particular questions you might have. =)