That's and the functionallity of SOUNDEX ist the reason. If you cast the collation like
select SOUNDEX('wah' collate Chinese_PRC_CI_AS); select SOUNDEX('wah' collate Latin1_General_CI_AS);
You get W000 as result.
Additional see SOUNDEX (Transact-SQL) : "string sounds when spoken in English."
Chinese words spoken in English ... I don't think you will get any result