PhotonVoiceNetwork and the SpeakerPrefab
It doesn't look like the SpeakerPrefab property is ever used in the PhotonVoiceNetwork class.
Pretty confusing as I expected it to create the prefab if I didn't have one in the scene.
Photon Voice v2.21 (September 8th, 2020)
It's used either by VoiceConnection.SimpleSpeakerFactory (outside of PUN integration) or by PhotonVoiceView.SetupSpeaker (in PUN integration).