I think it might have to do with this: the belt used for the Legend Rider Series Ryuki belt was also released back in 2004 as part of line just called "Rider Henshin Belt Series". They came out with Rider 1, V3, Kuuga, Agito, Ryuki (also with Knight & Ouja decks), Faiz, & Kaixa. All of those except the V3 & Kaixa ones have been retooled and re-released as the LRS belts.
Ryuki's is unique though because it's also originally based off the DX one, the LRS version being the fourth version of it (original DX, 13 Deck Toys R' Us repaint, RHBS version, and now LRS version.) Why it shares sound effects with the RHBS Kaixa belt... I have no idea! I guess it's something with the chipset like you said, but why those two belts, I don't know. It's pretty random.
Unfortunately I don't own the RHBS Kaixa belt or the RHBS Ryuki one so I don't know if either had similar thing, but I do know that all the sounds the LRS Ryuki belt makes are what the RHBS Kaixa's had.
BTW for reference this is what the RHBS releases looked like in the boxes, minus Kuuga:
http://www.mandarake.co.jp/information/2008/08/11/12cmp07/index.html
That's about the best theory I got. Hopefully that all made some sense. :laugh:
