When the sliders in the volume control are at a suitable level to minimize the internal sounds, it means I get an extremely small amount of travel on the speaker volume control. I can only turn it about 10-15 degrees.

Now *that* sounds like a mix-up of the Line level and Speaker (or headphone) level outputs. Sounds like the speakers are expecting line-level, but you're feeding them via a set of amplified outputs on the sound card.

Some sound card makers will cheat, and give you only one mildly-amplified output connector, suitable for driving headphones or small unpowered speakers. They expect you to use this for line level, too, which is just plain wrong (but they do it anyway).

On some sound cards, I get around this by plugging an in-line headphone volume controller into the connector, and attenuating the output before it reaches the speakers. This allows me to turn the sound mixer sliders up without overdriving the speaker inputs. I used to do this a lot in the old Doom days, back when Doom was a DOS application and you didn't have the sound mixer applet to adjust the soundcard volume. I discovered that if you cut back the volume slider within Doom itself, you actually started losing some of the more distant sounds on the map, it would actually reduce your "hearing range" in the game. So it was better to attenuate the volume in the analog domain (on your headphone wire) than from within the game.