I hear and understand my wife Linda's voice because I spend time with her talking and listening. The more familiar I am with her, the more I dwell with her, the more I am able to discern her voice from other voices.
The same goes for my relationship with God.
To hear the voice of God don't focus on hearing God, focus on your connection with God. Dwell in his presence. Abide in Christ. Out of the dwelling, abiding relationship we learn to recognize and understand the voice of God.
Hearing God is in proportion to intimacy with God.
Conversely, when intimacy is low discernment is low.
The growing ability to discern what God is saying in the prayer relationship comes from much conversational time with God. As Henri Nouwen writes: "Once we have committed ourselves to spending time in solitude, we develop an attentiveness to God's voice in us." (From Making All Things New, cited in Nouwen, Seeds of Hope, 64)