If you'd like to create a music-playing project, adding a Music Maker Wing will make it easy. These feature the VS1053 chipset which can do MP3, Ogg Vorbis, WAVE playback. They also have a MIDI synth inside that can be controlled over UART.
Both Music Maker Wings have the same circuitry/code but one has a headphone jack and the other has a 2 Watt audio amplifier. If you want to connect speakers directly, go with the amp. If you want to use headphones or connect to a powered stereo, the headphone one will work fine.
Main SPI Mode compatibility
There's a lot going on with these Wings, and they have some incompatibilities:
If you are only playing audio (MP3/Ogg/Wave), then these Wings are compatible with all Feathers.
But there are a lot of pins used: SPI + 3 for the MP3 chip and +1 for the SD card. MP3 playing works best with an interrupt, so those pins can be used at any time. We don't even recommend using this with Feathers that want to access SPI at their leisure, such as the Adalogger, non-nRF52 Bluefruit, or Radio. It is possible but use care to make sure you aren't using the two SPI devices at once.
We only recommend using this Wing along-side I2C-based FeatherWings to make sure there are no conflicts
MIDI Synth Mode compatibility
If you want to use the UART MIDI synth (which is not the same mode as the MP3/Ogg/Wave player), you'll need to use the UART TX pin. For the nRF52 or ESP8266 Feathers, you may need to remove the 'Wing during programming. And, you'll end up re-using the main Serial console for MIDI which can be confusing.
The Feather has the MP3 decoder chip and an SD card socket. You can stream MP3 directly to the chip if you like (say from a WiFi connection or SPI Flash if you have an Express) or read it from the SD card.
The headphone-out version has blocking capacitors so you can connect it to headphones or a powered stereo system.
The amplifier-out Wing is just like the headphone Wing but with a stereo Class D audio amplifier. This amp can draw quite a bit of current, 1A easily, if you're really bumping out some tunes! So make sure you have a beefy battery and/or 5V 2A wall adapter