I'm new to digital audio signals and read about [sampling rate on hydrogenaudio](http://wiki.hydrogenaudio.org/index.php?title=Sampling_rate).

What this doesn't mention is how *a single sample*'s value, ranging from -32768 to 32767 in a 16 bit, *relates* to any real-world amplitude. Since I sometimes use [replay gain](http://wiki.hydrogenaudio.org/index.php?title=ReplayGain), I guess this information is simply lost either in the AD conversion or in the data format. 

Also replay gain means to me that once there were a real-world amplitude and then there is an intended amplitude interpretation (aka replay-gain) and finally there is always an actual loudness value when the playback happens.

I saw that in many applications the amplitudes of wave files are expressed in negative dB values. I guess this is because there is a maximum amplitude value coming from the bits (and representation 'technique') used to represent the amplitude range, and the maximum amplitude possible in the file is taken as 0 dB.

Is my hypothesis regarding this right? 
How do I calculate the dBs of the rest of the samples once I decided that the maximum possible amplitude is "0 dB"? Does the minimum possible amplitude equal "silence"?
Are there file formats that store the original amplitude?