Morton Downey 11:15 Tues.-Thurs.-Sat. WINS-MBS. Morton Downey is back on the air-waves for his favorite soft drink at 11:15 P.M. three times weekly, on Tuesdays, Thursdays and Saturdays, over the Mutual Network coast to coast. In a program which is entirely different from the homespun songs and poems which he used to broadcast during the daytime, Downey is now specializing in what he calls his own kind of sooth-singing: soft, sentimental ballads and tunes. With Downey on his new program are a quartet of male singers who provide soft, melodic background for Downey’s silvery voice, and an intimate orchestra of eight under the skillful baton of Carmen Mastren. Born in Wallingford, Connecticut, the son of the local fire-chied, Downey is probably the Nutmeg State’s most famous good-will ambassador and most popular citizen. Nutmeggers remember him as the kid who used to sing at Elks’ benefits for nickels, accompanied by a friend who played the accordion. And they als