Optical audio transmitter can't handle speech
It seems to me that you are probably using a music source that is stereo and, instead of it delivering a mono signal as you expected (L plus R), the connection of the transformer to L and R channels results in L minus R.

Given that vocals and bass are usually equally mixed left and right in a stereo mix, the transformer connection you have used will only pass signals that are either in the left channel or the right channel hence, bass and vocal are largely ignored.