下面是一个示例代码，演示了如何在Android设备上响应有线耳机的音量减小操作：

首先，在AndroidManifest.xml文件中添加以下权限：

然后，创建一个BroadcastReceiver类来接收耳机插入和拔出的广播，并处理音量减小操作：

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.media.AudioManager;

public class HeadphoneReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();

        if (action.equals(Intent.ACTION_HEADSET_PLUG)) {
            int state = intent.getIntExtra("state", -1);
            int microphone = intent.getIntExtra("microphone", -1);

            if (state == 1 && microphone == 0) {
                // 耳机已插入，并且没有麦克风
                // 获取AudioManager实例
                AudioManager audioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
                // 获取当前音量
                int currentVolume = audioManager.getStreamVolume(AudioManager.STREAM_MUSIC);
                // 减小音量
                audioManager.setStreamVolume(AudioManager.STREAM_MUSIC, currentVolume - 1, 0);
            }
        }
    }
}

最后，在你的Activity中注册BroadcastReceiver：

import android.content.IntentFilter;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {
    private HeadphoneReceiver headphoneReceiver;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 创建BroadcastReceiver实例
        headphoneReceiver = new HeadphoneReceiver();
        // 创建IntentFilter实例，并添加对耳机插入和拔出的过滤器
        IntentFilter filter = new IntentFilter();
        filter.addAction(Intent.ACTION_HEADSET_PLUG);
        // 注册BroadcastReceiver
        registerReceiver(headphoneReceiver, filter);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // 取消注册BroadcastReceiver
        unregisterReceiver(headphoneReceiver);
    }
}

当耳机插入并且没有麦克风时，该代码将自动减小音量。你可以根据自己的需求进行修改和扩展。