From the Options in MMA you need to participate in the Beta program. Once done they'll be available from the Play store (and auto-update if that's your Play setting).

Where does one find the beta version?

Hi,
Do you still have the problem in recent 1.3.1 beta releases?

Which version of MMA, MMW and Android and was the card wiped as well?

Hi,
Did you get anywhere with this?
I have recently wiped my phone (LG G4) and am getting the same issue trying to re-sync all my music.

Hi there,

I just renewed my smartphone from Galaxy S5 to Galaxy S7.
I had MMA on old Galaxy S5 and wifi synced my tunes on the external SD card.
Now there is an error in wifi sync with my new S7 and PC.

On renewing my smartphone, what I did:
1. Pull out my SD card from my old Galaxy S5.
2. Put my SD card into my new Galaxy S7.
3. Install MMA on my new Galaxy S7.
4. Set up MMA on S7, including granting access to the external SD card.
5. Confirm to play songs on the external SD card with MMA on S7.
6. Wifi sync with S7 and PC, I got an error.

The error occurs as follows (after many reboots of S7 and PC, it still got the same):
1. Push 'Sync now' on MMA.
2. Initializing 'Synchronization is staring...'
3. Matching new files 'Checking for new files...'
4. After 2-3 seconds, Pop up message 'Sync incomplete'. Nothing on my PC.

There are so many music files on the SD card (>100GB), so I would like not to sync everything from blank.
Or is there any better way to switch to new smartphone?

Environments:
PC - Windows 7 SP1, MediaMonkey ver. 4.1.15.1830
old Galaxy S5 - Android 6.0.1, MMA ver. 1.3.0.0694 (this worked perfectly)
new Galaxy S7 - Android 7.0, MMA ver. 1.3.0.0694