Sound expansion cartridges with the Yamaha Y8950 sound chip (and upgrades for these cartridges).

Note that OPL1 is not at all an official name. But some people called it like that. It probably originates from the fact that the MSX-AUDIO (Y8950) is based on the Yamaha OPL (YM3526) with some extras, like the ADPCM channel. See MSX-AUDIO page for more information.