1、关闭/开启单一音量控制

frameworks/base/core/res/res/values/config.xml

    <!-- Flag indicating whether all audio streams should be mapped to
         one single stream. If true, all audio streams are mapped to
         STREAM_MUSIC as if it's on TV platform. -->
    <bool name="config_single_volume">false</bool>

开启单一音量控制后，系统音量控制将直接跟随媒体音量，由其单一控制；

2、单一音量控制流程解析

frameworks/base/media/java/android/media/AudioSystem.java

    /**
     * @hide
     * @return whether the system uses a single volume stream.
     */
    public static boolean isSingleVolume(Context context) {
        boolean forceSingleVolume = context.getResources().getBoolean(
                com.android.internal.R.bool.config_single_volume);
        return getPlatformType(context) == PLATFORM_TELEVISION || forceSingleVolume;
    }

frameworks/base/services/core/java/com/android/server/audio/AudioService.java

mIsSingleVolume = AudioSystem.isSingleVolume(context);

...

    private void updateStreamVolumeAlias(boolean updateVolumes, String caller) {
        int dtmfStreamAlias;
        final int a11yStreamAlias = sIndependentA11yVolume ?
                AudioSystem.STREAM_ACCESSIBILITY : AudioSystem.STREAM_MUSIC;

        if (mIsSingleVolume) {
            mStreamVolumeAlias = STREAM_VOLUME_ALIAS_TELEVISION;
            dtmfStreamAlias = AudioSystem.STREAM_MUSIC;
        } else {
            switch (mPlatformType) {
                case AudioSystem.PLATFORM_VOICE:
                    mStreamVolumeAlias = STREAM_VOLUME_ALIAS_VOICE;
                    dtmfStreamAlias = AudioSystem.STREAM_RING;
                    break;
                default:
                    mStreamVolumeAlias = STREAM_VOLUME_ALIAS_DEFAULT;
                    dtmfStreamAlias = AudioSystem.STREAM_MUSIC;
            }
        }

        if (mIsSingleVolume) {
            mRingerModeAffectedStreams = 0;
        } else {
            if (isInCommunication()) {
                dtmfStreamAlias = AudioSystem.STREAM_VOICE_CALL;
                mRingerModeAffectedStreams &= ~(1 << AudioSystem.STREAM_DTMF);
            } else {
                mRingerModeAffectedStreams |= (1 << AudioSystem.STREAM_DTMF);
            }
        }

    private final int[] STREAM_VOLUME_ALIAS_TELEVISION = new int[] {
        AudioSystem.STREAM_MUSIC,       // STREAM_VOICE_CALL
        AudioSystem.STREAM_MUSIC,       // STREAM_SYSTEM
        AudioSystem.STREAM_MUSIC,       // STREAM_RING
        AudioSystem.STREAM_MUSIC,       // STREAM_MUSIC
        AudioSystem.STREAM_MUSIC,       // STREAM_ALARM
        AudioSystem.STREAM_MUSIC,       // STREAM_NOTIFICATION
        AudioSystem.STREAM_MUSIC,       // STREAM_BLUETOOTH_SCO
        AudioSystem.STREAM_MUSIC,       // STREAM_SYSTEM_ENFORCED
        AudioSystem.STREAM_MUSIC,       // STREAM_DTMF
        AudioSystem.STREAM_MUSIC,       // STREAM_TTS
        AudioSystem.STREAM_MUSIC        // STREAM_ACCESSIBILITY
    };
    private final int[] STREAM_VOLUME_ALIAS_DEFAULT = new int[] {
        AudioSystem.STREAM_VOICE_CALL,      // STREAM_VOICE_CALL
        AudioSystem.STREAM_RING,            // STREAM_SYSTEM
        AudioSystem.STREAM_RING,            // STREAM_RING
        AudioSystem.STREAM_MUSIC,           // STREAM_MUSIC
        AudioSystem.STREAM_ALARM,           // STREAM_ALARM
        AudioSystem.STREAM_RING,            // STREAM_NOTIFICATION
        AudioSystem.STREAM_BLUETOOTH_SCO,   // STREAM_BLUETOOTH_SCO
        AudioSystem.STREAM_RING,            // STREAM_SYSTEM_ENFORCED
        AudioSystem.STREAM_RING,            // STREAM_DTMF
        AudioSystem.STREAM_MUSIC,           // STREAM_TTS
        AudioSystem.STREAM_MUSIC            // STREAM_ACCESSIBILITY
    };

AudioService获取到单一音量控制状态后，根据映射来更新系统不同音频流的音量；