missing headers?

You are brand new in the GNU/Linux world, aren't you? Welcome. :-)

Try to forget the painful way you used to install software on Windows (searching the application on the Web, downloading an installer, searching the installer, executing it, clicking "Next" a dozen of times, removing the useless entries in the menu, etc.).

Trisquel has a repository of free software that makes it very easy to install an application:

1. Run "Add/Remove applications" (to be found in the main menu);
2. Type the name of application you want (both Clementine and Mumble are present in Trisquel's repositories);
3. Check the boxes in front of them;
4. Click on "Apply changes";
5. Enter your password when queried (the user needs to be allowed to perform an administrative task).

A few minutes later the requested applications are ready to be used. :-)

with the add/remove software, I get the following message:

Some of the packages could not be retrieved from the server(s).
Do you want to continue, ignoring these packages?

the same message is displayed with synaptic

and using #sudo apt-get install mumble
returns the error #E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

wich I tried without sucsess

Problem solved with another clean install (same medium with correct md5sum, so no idea what the problem was)