Hi!
Bard Bard is a 750 RP skin, but part of a 750 RP skin includes all new model changes, and his meeps are models. Wild Card Shaco is a prime example of models changing, despite being a 750 RP skin - new clones, new JitB, etc. You are correct that we do something have QoL changes, such as Prehistoric Cho'Gath's Vorpal Spikes changing because of the model changes and we wanted cohesiveness there.
As for Sejuani Dawnschaser, since Bristle did change more drastically than other skins - she's riding a freakin' lion! - the sounds you're hearing are actually lion SFX, as opposed to the base SFX that Bristle has. May not seem totally obvious at first. Take another listen and lemme know if you can decipher them as lion (her joke is the best example, imo). :)