Intellectual Property, where is the border?

As there are no technical limitations to sharing any music, films, images or texts with everyone in the world, someone has to set up strict limit to what is legal to share - at least if we want to continue protecting intellectual property.

However, where can anyone find any natural border? Take the following cases:

• Play music for yourself in your walkman.
• Play music at home with your family.
• Play music at home with friends.
• Play music at home at a party with your colleagues.
• Play music at your workplace with your colleagues.
• Play music at your workplace for your customers.
• Play music at your workplace for your potential customers.
• Play music in marketing material for your customers.
• Send a music file to your sister in the next room.
• Send a music file to your brother in another country.
• Send a music file to a friend of your brother's in another country.
  
• As a music teacher, play someone's music for your pupils.
• As a music teacher, send someone's music file to your pupils.
• As a music teacher, post someone's music files for your current pupils on the web (with password check).
• As a music teacher, post someone's music files for potential pupils on the web (no password check).
• Send a music file to everyone within your corporation world wide.
• Post a music file for download by everyone within your corporation world wide.
• Publish an essay on a particular piece of music and illustrate it with written score snippets.
• Publish an essay on a particular piece of music and illustrate it with written scores that taken together cover the whole piece.
• Publish an essay on a particular piece of music and illustrate it with music sound samples.
• etc...
  

Which of them are legal, and which ones are illegal?

I'm sure lawyers and law makers have rules that can be applied to all those cases. However, where is the "natural" border between between legal and illegal? If there is no natural border, the law will be very difficult to apply. And it will be very different from one country to another.