ESPboy is an open source DIY handheld, kind of Arduboy, but powered with 32-bit ESP8266, and with a color screen. For sound it only features 1-bit piezo on the board (maybe will be replaced with a tiny speaker in further revisions), controlled via GPIO line. The creator of the thing asked me to help with the sound code, his intent is to bring ZX Spectrum beeper music to the platform. So I'll be porting some engines (Phaser1 is ported as proof of concept at the moment). Thought it may be interesting to some folks here.
More info on the device: