I have tried using the setSpeakerphoneOn() function both before and after initializing the Superpowered audio with new SuperpoweredAndroidAudioIO() as well as stopping and restarting Superpowered a...

Please check the latest Superpowered update (Q3 2017). It has an improved SuperpoweredAndroidAudioIO class, which has a stream type parameter now. If you pass SL_ANDROID_STREAM_VOICE, the speakerphone will be used.