Assuming you're on 4.0(x) you can use Audio Text Manager to do 15 minute schedule granularity - you can get the latest ATM for 4.0(3) or later here:
http://www.ciscounitytools.com/APP_AudioTextManager403.htm
Under the file menu, you'll see a "veiw schedules" option that lets you edit existing schedule to 15 minute chunks.
Be aware that if you make changes to the schedule in the SA, it will force it back into 30 minute chunks, so be careful about that.