背景

客户机器默认的开机声音一直很大；客户觉得无法接受，需要改小点。

基于Android 7的代码

前言

一般主要通过系统层来进行修改。

在系统关于音频的有关代码中，定义了两个数组：

注意，这些代码根据版本的不同而不同，但都能在frameworks/base/media/java/android/media中找到。
虽然我没有找到 定义音量最大值的入口，但是可以看看：https://blog.csdn.net/xuechwtx/article/details/80868996

• MAX_STREAM_VOLUME ：定义了各种声音的最大值。

注意：最大值不是100，需要AudioManager.getStreamMaxVolume(type)来获取各个音量的最大值，然后进行设置。

 private static int[] MAX_STREAM_VOLUME = new int[] {
     5,  
     7,  // STREAM_SYSTEMX_STREAM_VOLUMEMAX_STREAM_VOLUMEMAX_STREAM_VOLUME
     7,  // STREAM_RING
     15, // STREAM_MUSIC
     7,  // STREAM_ALARM
     7,  // STREAM_NOTIFICATION
     15, // STREAM_BLUETOOTH_SCO
     7,  // STREAM_SYSTEM_ENFORCED
     15, // STREAM_DTMF
     15  // STREAM_TTS
 };

• DEFAULT_STREAM_VOLUME ：分别对应各种声音的默认的大小（顺序与MAX_STREAM_VOLUME相同）。

public class AudioSystem
{
    private static final String TAG = "AudioSystem";
    /* These values must be kept in sync with system/audio.h */
    /*
     * If these are modified, please also update Settings.System.VOLUME_SETTINGS
     * and attrs.xml and AudioManager.java.
     */
    /* The default audio stream */
    public static final int STREAM_DEFAULT = -1;
    /* The audio stream for phone calls */
    public static final int STREAM_VOICE_CALL = 0;
    /* The audio stream for system sounds */
    public static final int STREAM_SYSTEM = 1;
    /* The audio stream for the phone ring and message alerts */
    public static final int STREAM_RING = 2;
    /* The audio stream for music playback */
    public static final int STREAM_MUSIC = 3;
    /* The audio stream for alarms */
    public static final int STREAM_ALARM = 4;
    /* The audio stream for notifications */
    public static final int STREAM_NOTIFICATION = 5;
    /* @hide The audio stream for phone calls when connected on bluetooth */
    public static final int STREAM_BLUETOOTH_SCO = 6;
    /* @hide The audio stream for enforced system sounds in certain countries (e.g camera in Japan) */
    public static final int STREAM_SYSTEM_ENFORCED = 7;
    /* @hide The audio stream for DTMF tones */
    public static final int STREAM_DTMF = 8;
    /* @hide The audio stream for text to speech (TTS) */
    public static final int STREAM_TTS = 9;

    // ...

    public static final String[] STREAM_NAMES = new String[] {
        "STREAM_VOICE_CALL",
        "STREAM_SYSTEM",
        "STREAM_RING",
        "STREAM_MUSIC",
        "STREAM_ALARM",
        "STREAM_NOTIFICATION",
        "STREAM_BLUETOOTH_SCO",
        "STREAM_SYSTEM_ENFORCED",
        "STREAM_DTMF",
        "STREAM_TTS"
    };

	// ...

    private static int[] DEFAULT_STREAM_VOLUME = new int[] {
        4,  // STREAM_VOICE_CALL
        7,  // STREAM_SYSTEM
        5,  // STREAM_RING
        11, // STREAM_MUSIC
        6,  // STREAM_ALARM
        5,  // STREAM_NOTIFICATION
        7,  // STREAM_BLUETOOTH_SCO
        7,  // STREAM_SYSTEM_ENFORCED
        11, // STREAM_DTMF
        11  // STREAM_TTS
    };
    // ...
}

如果需要修改默认的通知声音，则可以将对应的数值 改为新的值即可。

MIN：0为静音；MAX：每种音量都不一样。

思路很简单，只需要将对应的DEFAULT_STREAM_VOLUME改一下即可。

如何知道所要改的声音是属于哪种 type？这个可以查看有关的源码：

// frameworks/base/cmds/bootanimation/BootAnimation.cpp
void BootAnimation::bootMusic()
{
    int index;
    const char *fileName = "/system/media/boot.wav";
    MediaPlayer* mp = new MediaPlayer();
    // 从这里我们可以知道，是STREAM_SYSTEM_ENFORCED类型的
    audio_devices_t device = AudioSystem::getDevicesForStream(AUDIO_STREAM_ENFORCED_AUDIBLE);
    if (mp->setDataSource(NULL, fileName, NULL) == NO_ERROR)
    {
        mp->setAudioStreamType(AUDIO_STREAM_ENFORCED_AUDIBLE/*AudioSystem: :ENFORCED_AUDIBLE*/);
        mp->prepare();
    }
    LOGE ("bootMusic\n");
    
    AudioSystem::initStreamVolume(AUDIO_STREAM_ENFORCED_AUDIBLE, 0,7);
    AudioSystem::setStreamVolumeIndex(AUDIO_STREAM_ENFORCED_AUDIBLE, 7, device);
    AudioSystem::getStreamVolumeIndex(AUDIO_STREAM_ENFORCED_AUDIBLE/*AudioSystem::ENFORCED_AUDIBLE*/, &index, device);
    LOGE ("index %d",index);
    if (index != 0)
    {
        LOGD("playing %s", fileName);
        mp->setVolume(0.4f, 0.4f); // 调整左右声道的声音
        mp->seekTo(0);
        mp->start();
    }
}