Here is an open hardware portable music player that comes with everything you need to listen to songs, audio books, and podcasts. The Tangara comes with Bluetooth audio support. It has a 1.8-inch full color TFT display, two buttons, and a capacitive clickwheel.
The firmware for this is written in C++. You can always tweak it to further experiment. You can customize this audio player with custom faceplate. The device currently supports 16-bit audio at 44.1 or 48 kHz.