Androidのイヤホンの抜き差しを検知する ref: https://qiita.com/masaibar/items/649c85a86e3ad4ef7c97

HeadsetEventReceiver.kt

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

class HeadsetEventReceiver :BroadcastReceiver() {

    override fun onReceive(context: Context?, intent: Intent?) {
        if (intent == null || intent.action != Intent.ACTION_HEADSET_PLUG) {
            return
        }

        val state = intent.getIntExtra("state", AudioManager.SCO_AUDIO_STATE_ERROR)
        when(state) {
            AudioManager.SCO_AUDIO_STATE_DISCONNECTED -> {
                // 切断された時の処理
            }
            AudioManager.SCO_AUDIO_STATE_CONNECTED -> {
                // 接続された時の処理
            }
            else -> {
                // その他の場合の処理
            }
        }
    }
}

MainActivity.kt

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

class MainActivity : AppCompatActivity() {

    private lateinit var receiver: HeadsetEventReceiver

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        receiver = HeadsetEventReceiver()
        registerReceiver(receiver, IntentFilter(Intent.ACTION_HEADSET_PLUG))
    }

    override fun onDestroy() {
        unregisterReceiver(receiver)
        super.onDestroy()
    }
}