心声 (xīn shēng) refers to one's inner voice — the true, heartfelt thoughts and feelings a person holds inside. It differs from 心底 (xīn dǐ, 'the bottom of one's heart'), which focuses on depth of feeling, and from 心思 (xīn si, 'thoughts, mind'), which is more about one's current thinking or preoccupation. 心声 is often used when these inner feelings are expressed, e.g., in a song or a confession.