To do so, follow this tutorial: https://help.ubuntu.com/community/TextToSpeech
sudo apt-get install xsel
Create new file in your home folder, call it "clipboardtospeech.sh" or something.
edit it and paste the following line:
xsel --clipboard | festival --tts
Make the script executable
cmod +x ~/apps/clipboardtospeech.sh
Make the file executable make a hotkey that launches the shell script: