For powering external speakers. It's an output, not input.

Ah, thanks for the info. I figured out that the brand is MultiWave, but not entirely what model. It has a partnumber, MW50-316118-211 with 1.2 stamped or handwritten in ink beside.

I guess that would otherwise be known as "mwave"? I think cards like this were shipped in IBM systems and they powered the speakers that came bundled with it... they may have even been built into the monitor, I don't remember.

The computer in question is a Targa "FunLine". It has a Targa monitor, but no build-in speakers. Judging by the information I've found so far, this is a Multiwave Innovation AudioWave 16PNP FD, where FD means Full Duplex. I assume that's the properties of the ES1868F chip.