Why do I get PhotonVoiceSpeaker: Player underrun messages

I have a fairly simple Photon Voice setup with pretty much everything set to automatic, and it seems to be working nicely.

However, I get regular warning messages from PUNVoice, for example:

PUNVoice: PhotonVoiceSpeaker: player underrun: 161104/165360(164329) = 4256
UnityEngine.Debug:LogWarning(Object)
PhotonVoiceSpeaker:Update() (at Assets/PUNVoice/Scripts/PhotonVoiceSpeaker.cs:166)

What is causing it? What does it mean? and how should I fix it?

Thanks

Jules