Set *silence-mp3* to a valid path on my filesystem.

This prevents a typechecking error in sbcl when attempting to load the
code.
Rename the top-level package to pcl-practicals.

The filename had previously been changed from practicals.asd to
pcl-practicals.asd, but the package name was still just :practicals.

This makes it easy for me to (require :pcl-practicals) in sbcl and load
all the code from the book.