Unless it's a problem with hardware, which might not seem likely given that new speakers didn't seem to fix it (although, not being very knowledgeable of such things, it could well be a defect of some kind with your system's sound card, for all I know), in my experience Windows does make some of their sound controls somewhat obscure and more difficult to find than they should be. (Only assuming you're using Windows, of course, which you haven't specifically said.)
For example, while it's not the same problem you're having, I was trying to make a short webcam movie for a friend, and try as I might, I had a hell of a time trying to get the input volume from my microphone loud enough. In fact, after I sent her the video, she complained that as far as she could tell I wasn't saying anything at all, apparently unable to turn the volume on her system high enough to hear anything. In fact, I could only hear it myself at a level to understand what I was saying by turning my own system's sound up to the highest possible volume level.
In fact, I had turned my mike input volume up to it's supposedly highest setting with the settings control panel which comes up when I double-click click on the speaker icon in my Windows XP system tray, but as I said, even at the the highest volume setting I could set there, my recording was almost inaudible. Although in my case, unlike yours, other videos were of an "acceptable" volume level when played on my system.
But, through somewhat diligent scouring for other possibilities (including combing through various Windows help menus), somehow or other I eventually found my way to the "Sounds and Audio Devices" menu (Start > Control Panel > Sound and Audio Devices), where by clicking around the various menu tabs and buttons I managed to find a microphone input setting of some kind with which I think I was finally able to successfully increase the input volume of my microphone to an acceptable level, through a setting or option which, for some reason, didn't seem to be available in any way directly from my system tray sound icon.
While I don't know if your speaker problem may be solvable by going to the "Sounds and Audio Devices" menu and "poking around" looking at the various options and settings as I did, pending any more specific information or help here, if you haven't already done so, you might try doing the same and see if you can "stumble" on anything there which might help, as I eventually managed to.