RECENT news that veteran Canto-crooner Sam Hui has finally hung up his microphone has not gone down too well with one tai-tai of indeterminate age that we know. ''It is dreadfully sad news,'' moaned the senior swinger. ''No more Sam Hui in the flesh. And I had hoped so much to be able to hear him live on stage just one more time before either of us died.'' What a haunting thought.