Hello,
I have a combo PiZeroW + JustBoom Amp Zero pHAT with v2.378.
I have followed the tutorial found on the JustBoom website.
I had no sound when selecting JustBoom Amp Boards but when I select the JustoBoom DAC Boards it is OK 
I am not complaining because it works with this settings but they are very counter intuitiveā¦
I have the same setup, and initially also experienced the same.
But I believe I read somewhere that the I2C settings for the JustBoom board was only working for the Pi3 size board and not this small zero board.
I could be mistaken and have no larger AMP board.
But love the combination - and I use my second RP3 with volumio to fetch my files from so I like the small form factor of he PiZeroW combined with the Amp Zero 