默认音量
frameworks\base\media\java\android\media\AudioManager.java
/**  @hide Default volume index values for audio streams */
    public static final int[] DEFAULT_STREAM_VOLUME = new int[] {
        4,  // STREAM_VOICE_CALL
        7,  // STREAM_SYSTEM
        4,  // STREAM_RING
        8, // STREAM_MUSIC
        4,  // STREAM_ALARM
        4,  // STREAM_NOTIFICATION
        7,  // STREAM_BLUETOOTH_SCO
        7,  // STREAM_SYSTEM_ENFORCED
        11, // STREAM_DTMF
        11,  // STREAM_TTS
        4,  // STREAM_FM
        4  // STREAM_MATV
    };
最大音量
frameworks\base\media\java\android\media\AudioService.java
    private int[] MAX_STREAM_VOLUME = new int[] {
        6,  // STREAM_VOICE_CALL
        7,  // STREAM_SYSTEM
        7,  // STREAM_RING
        13, // STREAM_MUSIC
        7,  // STREAM_ALARM
        7,  // STREAM_NOTIFICATION
        15, // STREAM_BLUETOOTH_SCO
        7,  // STREAM_SYSTEM_ENFORCED
        15, // STREAM_DTMF
        15,  // STREAM_TTS
        13, //STREAM_FM
        13  //stream_MATV
    };