In rev B of the Featherwing, we connect the MIDI pin directly to the UART TX on the Feather without a pullup. For unknown reasons (the VS1053 is supposed to ignore the MIDI input when not in MIDI mode), having this pin floating sometimes causes a problem with decoding.
If you're having glitchy hangs, you can fix this intermediately by either:
1) Connecting a 10K - 100K pullup resistor between the Feather TX pin and 3.3V, highlighted here:
Set the Feather TX pin to be a pullup input or output level high.
On a Feather 32u4, Feather M0, or Feather Teensy, add either to setup:
On ESP8266 this is done by default so you can ignore this step (since the UART is always on). If using another kind of Feather, substitute 1 for whatever pin number is on UART TX.