Optical to analog - result: buzz

I need to use an Optical Audio Toslink Cable because my new TV does not have 'analog out'.  My receiver/sound system does not have Optical.  I have a converter: Optical to RCA.

However, the mP4 files I have listened to for the last few years are now only a buzz.  (I see the image on the TV) Is there a workaround?  Thanks.

Jim

(A big Audials fan.  There is no cable out in our rural area. I bring home things to watch from work where I have cable.)