[tep]

Technically, 0db is the loudest a digital signal can be. For the perceived loudness, however, the average loudness is what matters. If you have a recording of something that is loud for just a millisecond and the rest is rather quiet you would perceive the whole recording as quiet.

One way to come closer to a standard volume would be to establish a 'law' that says "no recording should be louder as x DB RMS). Every recording could be squeezed into such a dynamic, for example, by deploying compressors.

But there are two problems. 1) Different recordings need different frequency ranges. If you would compress classical music as hard as pop music is compressed, chances are the listening experience would be completely ruined. 2.) It's not just volume that influences how loud something is perceived. Material which has emphasis on 2khz would be perceived much louder than something that is just as loud but around 80 hz. (See Fletcher-Muson Curve[1])

However, there is a non-profit organization that somewhat tries to solve your problem. They don't aim for "standard volume". But they try to bring back dynamic into audio recordings.(Today most stuff has a very small dynamic range)

Read this: http://www.dynamicrange.de/en/our-aim

I hope I could help a little. Sorry for my poor English.

[1]http://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves

edit: Where are the guys from Ableton? Perhaps they could enlighten us :-)