Any way to make volume flat?

Yes... it's a very rarely addressed issue, but I've seen programs around called 'volume balancers' Ive never used them, so I cannot reccomend one over the other... but that should get you started on the google machine :0

I've been looking for ways to do this on WinAmp for years, and to date I haven't found anything that works properly. Either that, or it's so complex to set up I end up controlling the volume manually. If you do find something, let me know.

Well, the computer has no wattage (power) to drive the speakers. The multimedia speakers provide the 'power' or 'loudness source' if you will. But the sound chip (card or onboard) has a volume control (your volume control on the computer or card interface, not the speakers volume or 'power' control). Try turning the speakers (the wattage and sound power source) down to a minimum whwre you can comftorably listen to a movie. Then use the sound card or computer volume control to adjust the now 'low powered' volume level. This way the volume level has no 'wattage' to drive the loudness. Make sense. I do this with my 100 watt Marshall guitar amp all the time. I turn the amp up to get the 100 watts of power it is capable of producing cooking, then use the guitar's volume control to adjust the 'loudness' of the power source (the amp).

Bit of a cop out, but you could always wear heaphones. That's what I do when watching Formula 1 racing at unearthly hours of the morning.

Hmmm... Quite right, never though of suggesting that.

What if my computer speakers are connected to a receiver?

The way I have my sound set up is like this.

I have an old Aiwa CD player that connects to the speakers. I use this old aiwa cd player to connect directly to my sound card. Do I then just turn the sound on the AIWA cD Player all the way down till I can comfortably hear the movie, then just use the desktop volume control to adjust? Regarding the head phones; yes I use those for gaming, but when watching movies I usually sit back on the bed faraway. I nearly woke my sister up last night watching ninja assassins because of all the ridiculously loud fight scenes.

Do the speakers have a volume control? If so they provide power for the sound. Turn the power down on the speakers (or volume power source) to a comftorable listening level, not so loud. Then adjust the sound card's volume for loudness. The sound card has no wattage or power to the speakers you are adjusting upward. The speakers provide the volume's power. I am not familiar with the CD player, but unless it is a stand alone unit it has no wattage it produces. A 'receiver' would have wattage and need to be turn to a lower volume state, like don't crank up the 'power' of a 100-200 watt receiver for your purposes.

EDIT for clarification.

The best solution, and easiest, is to use headphones.

What you are looking for with the speakers is http://en.wikipedia.org/wiki/Dynamic_range_compression

The suggestions in the post will allow you to set up a good volume, but not adjust how loud the loud parts of a movie are over the lowest level of the low parts. So you will end up not being able to hear soft spoken sounds if you turn it down so the loud parts don't get annoying.

Unfortunately I have not been able to turn up a real-time tool for this, only ways to re-encode audio to enable this.

OP may try contacting the support techs of his sound card and ask for suggestions regarding his situation. Also, possibly send an email to other sound card support desks and ask if anything they have addresses the issue. I think it's the movie maker sound technicians using anything closely related to dynamic range compression for the creation of individual sound levels and the management of their presence on your DVD.

Wireless headphones. You can get some very reasonably priced ones nowadays.

Depending on what player you use for the movies, it may already have the options.
Go through the audio options looking for something called nightmode, normalize or something along those lines.

Options could be to use VLC player or Media Player Classic, possibly install FFDshow though that may be taking it a bit further than you would want/feel comfortable doing.

I think I found a quick way to do what I was trying to do. I just went into speaker properties then clicked on "enhancements" then check "loudness equalization"

This made it so that all sounds in the movies I'm watching are about the same. If there is background music playing in the film it will be louder than normal. If there is a loud explosion in the movie it will be quieter than normal.

I'm not sure if this will work for everyone because I have a realtek audio card, but it's worth a try.

You can go into speaker properties by clicking on the volume tab on bottom right hand corner, then click on the very top button once the menu comes up. Then go into enhancements tab and check, "loudness equalization".