Thanks Ben,
I think I managed to fix it. In case anybody finds this in the future, here is what I could see in the console:
[FMOD] PluginEffectModel::init : Missing DSP plugin ‘Resonance Audio Soundfield’
UnityEngine.Debug:LogWarning(Object)
FMODUnity.RuntimeManager:DEBUG_CALLBACK(DEBUG_FLAGS, IntPtr, Int32, IntPtr, IntPtr) (at Assets/Plugins/FMOD/src/Runtime/RuntimeManager.cs:36)
[FMOD] PostLoad::initPluginEffects : DSP plugin returned an error, bank will not load properly.
UnityEngine.Debug:LogError(Object)
FMODUnity.RuntimeManager:DEBUG_CALLBACK(DEBUG_FLAGS, IntPtr, Int32, IntPtr, IntPtr) (at Assets/Plugins/FMOD/src/Runtime/RuntimeManager.cs:32)
BankLoadException: [FMOD] Could not load bank ‘FMODAssets\Mobile/Master.bank’ : ERR_PLUGIN_MISSING : A requested output, dsp unit type or codec was not available.
FMODUnity.RuntimeManager.loadedBankRegister (FMODUnity.RuntimeManager+LoadedBank loadedBank, System.String bankPath, System.String bankName, System.Boolean loadSamples, FMOD.RESULT loadResult) (at Assets/Plugins/FMOD/src/Runtime/RuntimeManager.cs:703)
[FMOD] ObjectLookup::get : Lookup failed for EventModel: {821da335-a506-4dc6-a4a7-cefc5e90bff4}
UnityEngine.Debug:LogWarning(Object)
FMODUnity.RuntimeManager:DEBUG_CALLBACK(DEBUG_FLAGS, IntPtr, Int32, IntPtr, IntPtr) (at Assets/Plugins/FMOD/src/Runtime/RuntimeManager.cs:36)
EventNotFoundException: [FMOD] Event not found ‘snapshot:/Headphones’
FMODUnity.RuntimeManager.CreateInstance (System.String path) (at Assets/Plugins/FMOD/src/Runtime/RuntimeManager.cs:932)
[FMOD] ObjectLookup::get : Lookup failed for EventModel: {5765c19c-3e35-427d-973d-0c8ccb558a7e}
UnityEngine.Debug:LogWarning(Object)
So it looks like google resonance is screwing things up. I thought resonance was completely removed but it seems some remnants were still in fmod. I removed a couple of bypassed plugins in the fmod project and also the fmod plugin.
Re-built banks, re-started Unity and now I can play in Editor and have audio working!
Thanks for your help.