First you need to get all values, either with libjn:values
or its definition, then you can use fn:string-join
to get a single string:
So
declare namespace libjn = "http://jsoniq.org/function-library";
let $y := {"a":"welcome to the world of ","b":" JSONiq"}
return string-join(libjn:values($y) ! string(), "")
or
let $y := {"a":"welcome to the world of ","b":" JSONiq"}
return string-join($y() ! string($y(.)), "")
This might also return "JSONiqwelcome to the world of ", since the object keys are unordered