what is the diff between positive voltages and negative voltages??
Negatives are only used for audio amplifiers
Where did you get that idea? There are loads of things out there that require more than just a positive above common that are not audio. Radio? AC powerlines? Transformers for a variety of purposes, not just audio, power, and RF, either.
For us digitals, GND is the most negative we deal with.
Someone already flogged this with the example of ECL, but I'll also say that digital electronics must interface with the outside world. And the world is one big "return to zero" full of positives and negatives.
If you tied all Positive supplies together and call them GND, you have everything else more or less negative.
And had problems using NPN transistors the usual way
NPN bipolar and N channel JFETs and MOSFETs work just fine with a negative power supply with reference to ground. There is really nothing special about attaching the negative end of a battery or power supply to ground/common and the positive to a rail we call Power, it is just convention. Probably based on the aforementioned Benjamin Franklin guess about what charge the particles actually had, and people having an easier time thinking about current being supplied by the Positive end of a battery.