Software amplifiers don't work the way that electronic amplifiers do. There's no actual electroncal/analog signal to be amplified. Instead, what is being amplified are the digital channels in the your audio stream.
So your speakers are already preset to output a particular decibel range in response to audio channel data. That decibel range is determined by the hardware specs of your speakers and the volume settings. Therefore, along its output range of frequencies, it can only achieve X decibels of output no matter what input it receives. VLC Media player, FFdshow, or any other audio filters are unable to change this fact
So from the perspective of a software audio amplifier, all you can do is manipulate the sound channel to increase the perceptual or relative loudness of the audio channel within the fixed range determined by your hardware.
There are a few ways to do this:
- Adjusting the levels of the audio channel, much like an equalizer (a set of volume knobs for different frequencies that adjust the amplitude of that frequency(range)):
- The most basic way of doing this is cranking each frequency range's amplitude all the way up. You end up with a pretty flat frequency response and less-than-high-quality audio, but it increase the loudness of the audio channel compared to more reasonable/suitable equalizer settings.
- You can also use an equalizer setting that accentuates certain frequencies to the max while toning down other frequencies. You can get less of a boost this way, but it will usually sound better.
Adjusting the dynamic range of the audio
This is a legitimate technique that's unfortunately been abused in the past couple decades by music producers engaged in the loudness wars in the quest of producing a "louder" sound. Its basis is in the fact that audio formats have a fixed decibel range, e.g. CDs are governed by the redbook standard, which specifies how audio data is encoded onto the disc, as well as how playback devices should interpret the audio data, thus setting hard limits on how loud a CD can be.
However, the red book standard, like most modern multimedia formats, actually provides a very wide dynamic range to allow for hi-fi playback of music. It's capable of representing very quiet sounds, as well as extremely loud sounds, giving content producers the freedom of this huge dynamic range to play with.
Unfortunately, record producers discovered they could compress the dynamic range of their recordings (which naturally ranged from quiet to moderately loud to very loud notes) to just the upper part of the dynamic range scale, and their songs would blare out on playback devices much louder than they were actually recorded. This had the effect of making their artists seem more raw, exciting and "extreme", whilst making their competitor's albums seem tame and muted in comparison when they played much quieter on the same volume settings.
Well, your computer software can do the exact same thing. And it's actually a fairly common practice to do this when playing back DVDs, which are often mastered for the hi fidelity playback equipment of high end home theatres, causing the home PC or laptop playback experience to be less than ideal (gunshots and explosions being too loud, while dialog is too quiet).
Now, there are ways to do dynamic range compression that only increases the relative loudness of the audio, without undermining perceptual quality. However, if you over-compress the dynamic range, you will start to introduce audible distortions in the audio.
And then there are of course more advanced techniques that combine these basic methods with more intelligent algorithms and more novel manipulations that can "boost" the sound without deteriorating the quality. For instance, if you can over-compress the dynamic range of the audio while digitally compensating for the distortions it naturally creates, you can make the audio even louder while maintaining an acceptable level of quality. Supplementing the original bass signal using other frequencies can also produce a "louder" listening experience, without breaking any laws of audio electronics physics. Likewise, a smart algorithm might be able to pick out the sounds that audiences are interested in (dialog, music, central sound effects, etc. while muting noise or less important audio components).
long story short, there are myriad ways of amplifying the playback audio on your computer. Some are certainly more effective than others (VLC does a much better job than most filters), but there's no way it could itself damage your speakers unless your speakers were poorly designed or defective in the first place, since all they're doing is manipulating the audio channel to output it at the upper end of your speaker's natural output range.
Now, having your speakers blaring at 100% all the time may wear it out much sooner than playback at more reasonable levels, but that really has nothing to do with software volume amplification.