Hello, I have a problem where dialogue voices are not heard along with the sounds such as sheathing or unsheathing a weapon, or menu scroll noises... Not quite sure why it started doing this... any insight would be greatly appreciated...

Hi there. I too have experienced issues with Oblivion's rather sensitive sound processing. So the game's sounds were fine originally and they've just now started to play up?

Are you running any mods for your game? There may be errors from conflicting mods that could be affecting certain sounds, if you are, perhaps try running the game with all mods disabled.

Regardless if you are running mods or not, the first step I would take if you haven't already, is to find the latest version of drivers for your sound card/on-board sound and install them.