I was looking at the Lilypad MP3 and it seems that it would fit my project nicely, however I do have a question. I am looking to power 2x Lilypad MP3 with a lipo battery connect some small LED’s on the board, however I would also like to send serial commands to both units from a raspberry pi. Could I simply use a USB to UART serial connector on the FTDI side on the Lilypad and connect the USB end to one of the USB ports on the Raspberry Pi while the Lilypad is being powered by the Lipo battery?
Or should I be using a USB to UART serial with the USB on the Raspberry Pi side connected to the USB, and the TX pin on the USB converter connected to the RX(Trigger 5) on the Lilypad, and the RX pin on the USB converter connected to the TX(Trigger 4) on the Lilypad and GND going to GND?
Could I simply use a USB to UART serial connector on the FTDI side on the Lilypad and connect the USB end to one of the USB ports on the Raspberry Pi while the Lilypad is being powered by the Lipo battery?
This would be the easiest method to do what you’re after but you won’t need to power the Lilypad MP3’s with a battery if you don’t want too since the FTDI cable will be providing power from your Pi. 
Thanks Chris!
I think my earlier concern about the USB was how much power I would be pulling off the raspberry Pi but I believe that my Pi3B+ is 1.2A total for all 4 USB ports, and my LED draw for both Lilypads at the most will be 240 mA each.
Edit:
One thing I forgot. The documentation says:
The 5V FTDI port is provided to charge an attached Lipo battery, and provide a way to reprogram the board. It will power the board if no audio is playing, but will not provide enough power to drive the speakers (the board will reset while playing). In general, the best way to power the LilyPad MP3 Player is with a Lipo battery.
So does this mean if I connect the USB via FTDI with serial connection to the Pi, I won’t be able to play audio?
I can’t edit the above post but basically I am going to hook in a small NeoPixel LED Ring into the VBAT and GND where the Lipo battery connects. Since I will be using a Lipo battery I just need to ensure I can still use the FTDI to receive serial commands from my Pi and still be able to play sound with the FTDI connected with no issues since I am powering the Lilypad MP3 with the Lipo Battery but also connecting the FTDI via USB to my raspberry pi.
Oh, wasn’t thinking about the fact that those need the battery for playing MP3’s and external circuitry. You should connect a battery to each board and the Pi will recharge the batteries while the LEDs are not on and you’re not playing sound.
You only need about 500mA, a Pi should be OK with supplying that, just make sure your Pi’s power supply can handle the Pi + 500 extra mA and you should be OK.
Audio will still play even if you’re connected via FTDI/USB, the board doesn’t really care about that and doesn’t do any checking to see what’s connected.