There are 4 levels of voices in us.
1. From our mouths (verbal)
2. From our heads.
3. From our hearts
4. From our inner hearts.
What we say with our mouths would mostly be a reflection of what our head says, and sometimes what our hearts say. We may say with our mouths, what is not in our hearts.
What our head says, can very much be affected by how we are nurtured, what our situation is. We say in our heads things we think, is the right thing. However what we say in our heads, can still differ from what we utter with our mouths. I think this should be familiar to most.
And then there is what we say in our hearts. Heart is where conscious intention lies. When we intend to do something, it's in our hearts. Sometimes, we intend in our hearts to do an action - e.g. to do sunnah solat before isya - but our head may erroneously think to ourselves, 'I'm going to do my solat before maghrib'. - yet we immediately know in our hearts, that it's the wrong. Our hearts knew better.
For me, this is where the niyyah is. So even if you think wrongly with your head, it's your intention that is in your heart that matters. Purpose for an action comes from the heart.
Now the voices from the inner heart. The voices in the inner heart may be elusive, depending on the soul. The inner heart holds the truest part of us. It may lie hidden if we are not honest with ourselves. You can get to the real voices of this inner heart by digging in through self reflections. Ask the real reason, behind the 'real' reason, behind the reason for doing something or for 'believing' in something.
The hearts may still be forced by pride and greed to feel or think a certain way, even when our inner hearts are thinking the opposite. An example of this would be the case of Fira'un. He knows in his inner heart, that there is a God worthy of worship, but pride forces his heart to deny this.
From the above analysis, I find that the two most important voices are the inner hearts and the heart. In order to train ourselves to listen to our inner hearts, which contains the truth - is to apply humility to the heart, to purify the heart.
On that subject, I would recommend that we treat our hearts with God's words regularly, daily. Read the Quran and understand it's meaning. Make doa to Ar-Rahman (God- and Ar-Rahman is one of God's beautiful names) to guide us to the truest path.
Purification of the Heart - Translation and Commentary of Imam al-Mawlud's "Matharat al-Qulub" by Shaykh Hamza Yusuf, might also be helpful. Insyaa Allaah.
These words, are from my heart. :)
*update 17 Oct 2014.
Masyaa Allaah today, we covered tbis following ayat in tafsir class today.
As I read it literally, Allaah says the hearts really CAN learn wisdom! I truly believe that. Subhanallaah.
أَفَلَمْ يَسِيرُوا فِي الْأَرْضِ فَتَكُونَ لَهُمْ قُلُوبٌ يَعْقِلُونَ بِهَا أَوْ آذَانٌ يَسْمَعُونَ بِهَا فَإِنَّهَا لَا تَعْمَى الْأَبْصَارُ وَلَٰكِن تَعْمَى الْقُلُوبُ الَّتِي فِي الصُّدُورِ
"Do they not travel through the land, so that their hearts may thus learn wisdom and their ears may thus learn to hear? Truly it is not their eyes that are blind, but their hearts which are in their breasts."
Surah Al-Hajj, 22:46
(English translation by Yusuf Ali via iQuran)