Ah, I missed the hissing part. Apparently he gets that noise only from a CD player, and that noise is constant through all song and unaffected by volume settings. That would mean the audio section of that player is really noisy and/or possibly the volume control has been put in the wrong place, for example too far (circuit wise, not distance) from the power amplifier section. When connecting multiple stages together care must be taken to minimize noise. Besides careful design and choosing low noise parts, some practices can at least reduce the perceived noise even on noisy devices, for example by putting the volume control just before the power amplifier. This ensures that when the user lowers the volume the noise generated by previous stages gets also lowered along the signal.