Click the 'Customize Quick Access Toolbar' arrow at the right side of the Quick Access Toolbar.
Choose 'More commands'
In the 'Choose commands from' drop box, set it to 'All commands'
Go down the long list of commands and add the 'Speak cells' command.
Then you can click the Speak Cells button in the Quick Access Toolbar and it will read whatever text is in the cells.
By the way, for future reference, I found that by opening Excel 2007's help and searching for speech. Usually help files are even more helpful than forums.