This video is an in-depth video guide to assembling the new Soulsby Synthesizers miniAtmegatron synth shield for Arduino Uno. 

The miniAtmegatron turns the Uno into an 8-bit synthesizer with an audio engine based on the Atmegatron. The package comes with instructions, PCB and all the electronic components. The Arduino Uno must be bought separately.

The second video, below, looks at adding MIDI support:

The miniAtmegatron is available as a DIY project for $39 / €39 / £29. An Arduino is also needed. See the Soulsby site for details.