Currently running OSR2.5 C.
Did all of the above recommendations, with no change unfortunately.
DirectX actually wasn't installed, so I installed it, but no change.
Interestingly, it appears that the only sounds that jitter are system sounds and WAV files. Playing video files (for example, from the FUNSTUFF folder) leads to almost flawless playback, hold for the few pauses in playback.
UPDATE: Tried it on Windows 98 SE as well, with no change. In fact, Windows 98 seems to have even more issues, as it crashes with an empty white box before it can even load the desktop.