A decibel

There's a misunderstanding of what a dB is and what it measures. Here's a definition from Sweetwater's word for the day archive:

• A decibel (named for Alexander Graham Bell) is a tenth of a bel, and is used as an expression of power. Here's where the confusion arises: A decibel isn't a measure of ANYTHING; it is a ratio of two power levels. Because of the way our ears perceive volume, these ratios follow a logarithmic curve, expressing them as a decibel keeps things easier to deal with. Here are a few convenient decibel figures worth remembering: One decibel is commonly taken as the smallest volume change the human ear can reasonably detect. Doubling the POWER of an amplifier results in a 3 dB increase, which is a "noticeable" volume increase. Doubling the VOLUME of a sound is a 6 dB increase (you may occasionally see 10 dB listed as the "double-volume" figure, 6 dB is the more mathematically correct number). By doing the math, you can see that truly doubling your volume actually requires 4 times the amplifier power! Keep these figures in mind the next time you are comparing the specs of two pieces of equipment...
  
  Decibels are used all over the place in audio measurements and specifications partly because it is very easy to express extremely large or small values with decibels due to their logarithmic nature. The full depth of the topic is well beyond the scope of this writing, but suffice to say that the decibel always refers to a ratio of two values. It is never an absolute value. Thus when we speak of losses and gains in audio we use dB to quantify those values. If a signal goes into a unit and is output at a lower level it is said to be down by some value of dB. Decibels come in many variants: dB PWL, dB SPL (also known as Sound Pressure Level), dBm, dBu, dBV, dBv, and more.

The +/- 12 dB slider in WinAmp is a representation of dBV. Volume is measured in the air as dB SPL. The -80 to zero dB scale you referred to is dBFS (dB Full Scale). Because a dB is a ratio, and not an absolute unit of measure, it can be used to measure voltages or sound pressure levels in any number of different ways.

Without knowing what you are using as an output device (headphones, speakers, an amplifier with huge speakers attached, etc.) there is no way the software could know what SPL level you are producing, or that the MP3 player could know when you've exceeded a safe threshold (although it could make certain assumptions since most people listen to MP3 players with earbud-style headphones).

There's no way to use your webcam mic to measure SPL. You'll need an SPL meter with a built-in microphone or a calibrated mic that outputs a known voltage at a certain SPL.

dB FS is a measurement of the level of a "word" in digital audio. A "digital word" is a string of either 1's and 0's (usually 16 or 24 of them) which define a sampled sound's volume at a moment in time. Dealing with 16-bit audio, specifically, there is a theoretical maximum range of 96dB.

A -96 dBFS word would be 16 zeros, the smallest value a 16-bit word can represent:
0000000000000000

a 0 dBFS word would be 16 ones, the largest value a 16-bit word can represent:
1111111111111111