session-android/res/drawable/webrtc_speaker_button.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/webrtc_control_background"/>
    <item android:top="5dp"
          android:left="5dp"
          android:right="5dp"
          android:bottom="5dp"
          android:drawable="@drawable/ic_volume_up_white_24dp"/>
</layer-list>