Since /splay also does not work with sounds off, I would mostly agree with this.
That said, the argument could be made that the usage of /speak has a slightly different purpose than typical sounds, since it is partially intended for accessibility concerns. Under that argument, /speak isn't really an aesthetic sound so much as it is an integral part of the experience, for some users. If this is the case, then it was probably left on intentionally, but I'm not sure.
