It's not serious, but it is a bit immersion-breaking to pause the game briefly to quaff a potion next to an Oblivion Gate, only to result in the Gate going completely silent, or checking the map next to a waterfall resulting in the waterfall's sound to stop playing.
Any sound that is emitter-based is subject to this (be it if the sound is emitted by an actual mesh or if it is using a marker-based emitter). Footsteps, weapon noises, region sounds, etc. are not subject to this.
Sound card is a Creative SB X-Fi Titanium PCI-E
I have taken Oblivion off of my ALchemy list because ALchemy causes quality loss to occur with certain sounds (of all things). Since Oblivion doesn't support EAX effects, though, I'm not losing much by doing this. So yes, I am not hardware-accelerating sound for Oblivion (Oblivion seems to have issues with ALchemy for me).