先看最终需要修改哪里：
framework层通过jni最终会调用到AudioPolicyManagerBase.cpp中的checkAndSetVolume（）函数
路径：\hardware\libhardware_legacy\audio\AudioPolicyManagerBase.cpp
其中的volume是放大比例：从0.00 ~ 1.00

status_t AudioPolicyManagerBase::checkAndSetVolume(int stream,
                                                   int index,
                                                   audio_io_handle_t output,
                                                   audio_devices_t device,
                                                   int delayMs,
                                                   bool force)
{
………………

//float driverVol[6]= {0.00,0.02,0.03,0.04,0.05,0.06};
    //float volume = computeVolume(stream, index, output, device);
    //if( index < 6)
    //{
    //    volume = driverVol[index];
    //}
    float volume = 0.20;
    float driverVol[16]= {0.00,0.10,0.20,0.30,0.40,0.50,0.55,0.60,0.65,0.70,0.75,0.80,0.85,0.90,0.95,1.00};
    if( index < 16)
    {
        volume = driverVol[index];
    }
    ALOGD("AudioPolicyManagerBase::checkAndSetVolume %d,%f",index,volume);

    // We actually change the volume if:
    // - the float value returned by computeVolume() changed
    // - the force flag is set
}

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