I finally figured out the echo/reverb problem.

On a sudden hunch, I went into the Control Panel and clicked on Sound Effect Manager. Turned out the "Environment" controls had been set to "Auditorium" instead of "None". I'm not really sure how that could've happened in the first place -- I hadn't messed with the sound at all since formatting my hard drive last week -- but at least it's fixed now.

Many thanks for all your help, gentlemen.