With ISA based sound cards, or even the appropriate USB ones on a dedicated channel, it should be possible to trick Windows OS and get real-time microsecond performance, however, it would only be in your own app, there would be no windows system audio without extra programming and 1 cpu core would be pretty much eaten alive stuck at around 10% usage all the time. Though, you would never need to worry about buffer under-run errors if programmed properly.
Note, 5 years ago, In Win7 64 bit, I had a test display rendering tool where I manually swapped the video-cards full graphics screen buffer timed to one of the system's precision timers which operate in the microsecond range. I was able to swap page buffers on specific lines of video on the screen with other tasks running. Meaning, a valid Window app can time and interrupt to call system functions and control connected hardware with accuracy down to a 65Khz line of video without fail. With this, you can make an audio driver which responds within 2 to 4 audio samples if you care to write your own sound driver.
So, there is no need for a real-time OS to make a super fast/short sound buffer sound driver.