Texas Instruments SN76489AN which was used on the Sega SG-1000 game console. It has one feature missing - it does not automatically switch between analog and s/pdif output. Necessary because of those Boston Acoustic speakers that had S/PIDF over a miniplug connector.

Is there an SN76489ANA that is Sega branded? From their SC-3000 computer?

It is an incidental fix from modifying the chip to handle "PC speaker" mode.

What OS did it use? Was it Dr-DOS 6.28?

I think there was a SN76489ANBA ASIC that was Edirol branded that also handled system error messages.

The SN76489ANBTA Atlona branded ASIC that was never issued is the final solution that fixes compatibility with the Edirol and Yamaha synthesizer ASICS.

In other news, the name on the synthesizer ASIC on the original Gameblaster card is "soundblaster."