Neat idea. OS X's clock can be configured via System Preferences to announce the time on the hour, half-hour, or quarter hour, but not every 5 minutes. For that, save the program below in $HOME/bin/saytime:

    #!/usr/bin/perl
    use POSIX;    
    system "say", "It is now " . POSIX::strftime("%I:%M %p", localtime);

Then add the following line to your crontab:

    */5 * *  * * $HOME/bin/saytime