首先，修改返回的StreamType。默认的是DEFAULT_VOL_STREAM_NO_PLAYBACK，这里定义为AudioSystem.STREAM_MUSIC，可以直接修改DEFAULT_VOL_STREAM_NO_PLAYBACK的定义，也可以直接返回AudioSystem.STREAM_RING。

//frameworks/base/services/core/java/com/android/server/audio/AudioService.java
    private int getActiveStreamType(int suggestedStreamType) {
    ...........
            } else if (suggestedStreamType == AudioManager.USE_DEFAULT_STREAM_TYPE) {
    ...........
                    return AudioSystem.STREAM_RING;//DEFAULT_VOL_STREAM_NO_PLAYBACK;
    ...........
}

当声音为0时，system ui 的bar上添加一个铃声为0的图标。

AndroidManifest中添加定义。

//frameworks/base/core/res/AndroidManifest.xml
    <protected-broadcast android:name="android.intent.action.SILENT" />
    <protected-broadcast android:name="android.intent.action.NORMAL" />

PhoneStatusBarPolicy中监听。

//vendor/mediatek/proprietary/packages/apps/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBarPolicy.java
    public void init() {
        // listen for broadcasts
        IntentFilter filter = new IntentFilter();

        filter.addAction("android.intent.action.SILENT");
        filter.addAction("android.intent.action.NORMAL");

        filter.addAction(AudioManager.ACTION_HEADSET_PLUG);

        filter.addAction(AudioManager.ACTION_BASEMCU_PLUG);
        ...........
      }



        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();
            switch (action) {
                case "android.intent.action.SILENT":
                    Log.d(TAG,"SILENT");
                    mIconController.setIcon(mSlotVolume, R.drawable.stat_sys_ringer_silent, null);
                    mIconController.setIconVisibility(mSlotVolume, true);
                    break;
                case "android.intent.action.NORMAL":
                    Log.d(TAG,"NORMAL");
                    mIconController.setIconVisibility(mSlotVolume, false);
                    break;
                case Intent.ACTION_SIM_STATE_CHANGED:
                    // Avoid rebroadcast because SysUI is direct boot aware.
                    if (intent.getBooleanExtra(Intent.EXTRA_REBROADCAST_ON_UNLOCK, false)) {
                        break;
        ...........
      }

按下声音按键 后，会交过VolumeDialogImpl去处理。把STREAM_MUSIC的row换成STREAM_RING。

//vendor/mediatek/proprietary/packages/apps/SystemUI/src/com/android/systemui/volume/VolumeDialogImpl.java
            addRow(AudioManager.STREAM_MUSIC,
                    R.drawable.ic_volume_media, R.drawable.ic_volume_media_mute, true, true);
            if (!AudioSystem.isSingleVolume(mContext)) {
                addRow(AudioManager.STREAM_RING,
                        R.drawable.ic_volume_ringer, R.drawable.ic_volume_ringer_mute, true, false);

改成

            addRow(AudioManager.STREAM_RING,
                    R.drawable.ic_volume_ringer, R.drawable.ic_volume_ringer_mute, true, true);
            if (!AudioSystem.isSingleVolume(mContext)) {
                addRow(AudioManager.STREAM_MUSIC,
                        R.drawable.ic_volume_media, R.drawable.ic_volume_media_mute, true, false);

    private VolumeRow getActiveRow() {
        for (VolumeRow row : mRows) {
            if (row.stream == mActiveStream) {
                return row;
            }
        }
        for (VolumeRow row : mRows) {
            if (row.stream == STREAM_MUSIC) {
                return row;
            }
        }
        return mRows.get(0);
    }

STREAM_MUSIC改成STREAM_RING。具体如下。
if (row.stream == STREAM_RING/*STREAM_MUSIC*/) {

最后,当音量为0时发送广播。

//vendor/mediatek/proprietary/packages/apps/SystemUI/src/com/android/systemui/volume/VolumeDialogImpl.java
    private void updateVolumeRowH(VolumeRow row) {
        .................
        updateVolumeRowSliderH(row, enableSlider, vlevel);

        if (isRingStream){
            if(vlevel == 0){
                Intent intent = new Intent();
                intent.setAction("android.intent.action.SILENT");
                mContext.sendBroadcast(intent);
            }else {
                Intent intent = new Intent();
                intent.setAction("android.intent.action.NORMAL");
                mContext.sendBroadcast(intent);
            }
        }
   }