A bunch of TTL chips playing the "anthem of 8-bit computing". Four channels: 2x pulse, 1x triangle, 1x noise
The player is an enhanced version of the Contrib/at67/midi player in the gigatron-rom repository. The noise is created from waveform 0 ("metallic") that receives a randomised XOR modulation 60 times per second.
Only 7400 logic. No microprocessor, no video chip, no sound chip.
The Arduino is used to load the music and player into RAM. You see that happen in the first few seconds of the video.