[mhartl]

In OS X, you can do this:

1. Copy the text of the talk and save it to a file, say "life.txt"

2. Use the "say" command to read it:

    $ say < life.txt

To change the voice, go to System Preferences > Dictation & Speech and pick the one closest to Hawking's. (On my system, "Fred" comes pretty close.)