When he talks about his favourite subject, the tone of his voice changes
「声のトーンが変わった」＝ The tone of his voice changed / He changed the tone of his voice / When he talks about his favourite subject, the tone of his voice changes
※女性の場合、「he」ではなく「she」にしてくださいね＾＾
ボキャブラリー
tone of 人's voice ＝ 声のトーン
changed ＝ 変わった
when ＝ ～の時
talk about ＝ ～について話す
favourite subject ＝ 好きな話、好きなテーマ
イギリス 
