Is there any other voice in a person’s heart apart from that of his conscience?

All of the other alternatives that surface after the voice of one’s conscience are the “voices of the self” that try to suppress the conscience. The self tries with all its might to prevent one from behaving righteously and to make one do evil deeds. 

It may not do this very openly. It may outline certain excuses which may seem sensible. It may cause one to say “all this will come to nothing.” Allah states this truth in the Quran:

“By the soul and Him that moulded it and inspired it with the knowledge of sin and piety: blessed shall be the man who has kept it pure.” (Surat ash-Shams: 7-9)

As stated in the verse, man is a prey to sin but knows he has a duty to guard against it. Man is tested on his choice between good and evil.