With a simple solder jumper connection, the board will boot up in MIDI mode that will read 'classic' 31250Kbaud MIDI data on the UART pin and act like a synth/drum machine - there are dozens of built-in drum and sample effects.
On the Feathers that have a separate UART for USB communicating (e.g. 32u4, M0, WICED, Teensy and any others with native USB) you'll want to use Serial1 as the serial port. On the ones that use the UART (e.g. nRF52, ESP8266, etc) use Serial and note that this will make the serial console not work right because the baud rate is now 31250 KBaud!
You may also need to remove the FeatherWing while programming boards that have Serial not Serial1 as the serial port because the VS1053 can interfere with the UART during upload. If you have a 32u4, M0, Teensy, etc with native USB you will not need to remove it.
By default, MIDI mode is not 'activated' - but its very easy to turn on. Start by soldering the jumper on the bottom of the Feather.
Once that is done, you can no longer play an MP3, you will only be able to send MIDI commands via the hardware UART (TX) pin. That pin is automatically connected.
Now load up the MIDI demo File->Examples->Adafruit_VS1053->player_miditest
Upload to the Arduino + Shield and listen on the headphone jack for the Ocarina scale being played. You can check the datasheet for a list of all the instruments (there's a lot!)