If you want to listen to it again you have to type the following commands:
sudo modprobe pcspkr
sudo apt-get install beep
If you want to make the change permanent you must edit the file /etc/modprobe.d/blacklist.conf and comment out the line that disables the internal speaker:
blacklist pcspkr
Now you get:
# blacklist pcspkr
Now you can enjoy the shrill and charming sound of the internal speaker again ;-)