For me it's either "mute" "less loud and "a little more loud", stance that can be applied in two clicks starting from mute. Tuning the volume up has no further effect.

What ossipena is trying to say is that your output is not amplified. If you were a techy kid you should know there's no party without an amplifier. To test it simply connect an mp3 player to a speaker directly and you'll see the db output is incredibly small.

More multimedia oriented devices such as dvd player may have a built in amplifier, however this is very rare.
Even connecting your pc to a "rough" speaker won't yield any useful output. Remember that if your speaker plugs in (aka. is powered) it means it has a built in amplifier.

Amplifiers require quite some power and increasing the output volume on N900 is useless since your car already has an amplifier that can boost the volume more than enough (mine's @ 25%, everything above is LOUD to cracking your ear drums).

Hope this clears things up.