So koz has already answered this, but this is soch a juicy question 
The important word in the above statement is “seems”. These effects are entirely down to the playback system.
In my house, audio CD’s are much louder than tapes !
Why is this?
It’s because my cassette player is attached to a domestic 30 watt per channel hi-fi system, whereas I have a CD player that is connected to a 2500 watt per channel PA system. The tapes I can play loud, and with CD’s I can make your ears bleed.
So this may not be exactly what was meant, but the fact is that recordings do not need to be “maxed out” in order to sound loud, most playback systems include a volume control that can take care of that. If your play back system will not go loud enough with a well recorder source, then you need a more powerful playback system.
So why is it that recording “A” sounds louder than recording “B”, even though both recordings go full scale in Audacity tracks? (Technospeak: they are normalized to 0dB. Non-techy- speech: the peaks reach all the way to the top and bottom of the track).
This is where we nip over to Wikipedia and look up "Digital Audio: http://en.wikipedia.org/wiki/Digital_audio
As we see from their illustration, our digital recording aims to accurately trace the recorded waveform, but is only able to approximate to it:
Furthermore, representing a waveform in number this way means that there is an absolute maximum value that can be used for a sample, and an absolute minimum value.
Since CD recording use “16 bit” binary digits, that effectively gives us a maximum range for any sample of between 0 and 1111111111111111, which in normal numbers (decimal) is a range of 0 to 65535, but since our waves go up and down (zoom in close on an Audacity track to see the wiggly blue line), this gives us a range from “minus 32,757” to “plus 32,767”.
A waveform that moves between these two extremes is at the maximum possible amplitude, and for that specific frequency, nothing can go louder. This is called th zero decibel level (0 dB). On a well designed system, this would equate to the “red line” on our level meters.
With our Digital recording, the quietest possible signal is a flat line (usually at “0”, or close to it) - this is silence.
The quietest sound we can record would be a signal that fluctuated between a high and low value that cover a range of “1” - for example, fluctuating between 000000000000000 to 000000000000001 (which would be very quiet indeed).
So on CD’s we have a maximum possible range that goes from, silence to 0dB in 65,535 steps.
We said earlier that digital audio tries to copy the shape of the sound wave as precisely as possible. Any variation from this is some kind of distortion. Is distortion a bad thing? Well Jimmy Hendrix would say no. Distortion can sometimes produce a more desirable sound than a completely clean and undistorted sound. On the other hand, we have all heard recordings that are badly distorted and sound terrible. It all depends on the kind of distortion, and whether it is appropriate (desirable) for our recording.
So how do we make a recording sound louder? We can use a “trick” called “dynamics compression”. We have a maximum range of loud to quiet, but we can apply a special kind of distortion (called “dynamics compression”) where we make more of the sound reach the highest levels. Effectively we “squash” (compress) all the sound into the louder part of our dynamic range. This kind of “compression” has nothing to do with “compressed formats” (such as mp3’s) - This is compressing the dynamics (loud / quiet ratio) whereas “file compression” (mp3’s) are about making the file size smaller.
The Good thing about dynamic compression - it can make music sound louder when played at a specific volume level. It can even out volume fluctuations that can sometimes be desirable. It can help make different tracks on an album sound at a similar volume level.
The Bad thing about dynamic compression - it can make music sound “flat” and monotonous. It reduces the dynamic contrasts in the music.
The Ugly thing about dynamic compression - it is a form of distortion and it is throwing away some of the sound quality that we can have. It assumes that people are too stupid or too lazy to adjust the volume control on their CD player. In extreme cases it will turn all of our lovely musical accents into flat farty sounds.
OK I’ve had my 10 cents. There’s a certain amount of personal opinion and personal taste involved when it comes to compression, but hopefully this provides a little insight into what it’s all about.
Back to the original question - you missed out FLAC (lossless) - approx 50% lossless compression, but probably not compatible with iPods. Best quality on iPods is probably using Apple Lossless - not because it’s any better than other lossless compresors, but being Apple, it will be well supported on iTunes and iPods.
There’s a load more information about audio file compression codecs here: http://en.wikipedia.org/wiki/Comparison_of_audio_codecs
