Comment by peterfirefly

> had essentially their own processors to handle this stuff

Most of the benefit came from the sound card doing DMA from a memory buffer instead of using a high-frequency interrupt to have the CPU give it sound samples one or two bytes at a time (fetched from the very same memory buffer(s)).

More sophisticated cards could also do mixing of several sounds in "hardware" (usually by having their own little CPU do the work) instead of having the main CPU doing the mixing. Some of them could even play the constituent sounds at different speeds (Gravis Ultra Sound and Midi cards but the Midi cards usually had fixed sound samples). This mattered for more sophisticated game audio for a few years until x86 performance improvements made it irrelevant. DMA is still crucial.