Increase or decrease the audio volume of Ringtone, Notifications, Music, Video and Alarm from the Sound setting. To set and change audio volume on HTC One do as follows:
1. Open "Settings" app
2. Scroll down and tap "Sound"
3. Tap "Volumes" to set audio volume
4. Drag right or left to increase or decrease the audio volume for: Ringtone & Notifications, Music & Videos, Alarms