Aparentemente parece esta tudo certo,

Suspeito que é o emulador

Fiz o teste no Emulator Control com um numero NÃO cadastro e só apareceu a mensagem e fiz o teste com numero cadastrado apareceu a mensagem e o toast, MAS não ouvi o som.

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.media.MediaPlayer;
import android.telephony.SmsMessage;
import android.widget.Toast;
import br.com.caelum.cadastro.dao.AlunoDAO;

public class SMSReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {

        Object[]mensagens    =(Object[])intent.getExtras().get("pdus");
        //ultima sms , de índice 0
        byte[] mensagem=(byte[])mensagens[0];

        // Classe que representa o SMS        
        SmsMessage sms = SmsMessage.createFromPdu(mensagem);
        String telefone = sms.getDisplayOriginatingAddress();

        AlunoDAO dao = new AlunoDAO(context);
        if(dao.isAluno(telefone)){
            Toast.makeText(context, " SMS de aluno "+telefone, Toast.LENGTH_LONG).show();
            MediaPlayer mp = MediaPlayer.create(context,br.com.caelum.cadastro.R.raw.msg);
            //mp.start();

        }
        dao.close();
    }

}

Manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="br.com.caelum.cadastro"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="21" />

    <uses-permission android:name="android.permission.CALL_PHONE" />
    <uses-permission android:name="android.permission.READ_SMS" />
    <uses-permission android:name="android.permission.SEND_SMS" />
    <uses-permission android:name="android.permission.WRITE_SMS" />  
    <uses-permission android:name="android.permission.RECEIVE_SMS" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

        <activity android:name=".ListaAlunosActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".FormularioActivity" >
        </activity>
        <activity
            android:name=".DouglasActivity"
            android:label="@string/title_activity_douglas" >
        </activity>

        <receiver android:name="br.com.caelum.cadastro.receiver.SMSReceiver" android:enabled="true">
            <intent-filter>
                <action android:name="android.provider.Telephony.SMS_RECEIVED" />
            </intent-filter>
        </receiver>
    </application>

</manifest>