I’ll interpret this as:
Because QSound was a proprietary technology licensed by Capcom, the MAME team cannot legally package the necessary firmware files (like dl-1425.bin
QSound was a revolutionary "3D" audio technology used heavily by Capcom in their CPS-2 (Capcom Play System 2) arcade boards. It allowed stereo speakers to produce a wider, more immersive soundstage. In the world of emulation: stands for High-Level Emulation
(default is dsp for low-level emulation)