One more option:
ReRemind
Project Description
ReRemind is a Windows Mobile service that plays a brief sound at a specified interval when reminders, missed calls, and voicemails are awaiting your attention.
Service
ReRemind runs as a true Windows Mobile service -- it is a DLL hosted by services.exe. Therefore, ReRemind does not use one of the very few 32 processes available. ReRemind is also very light on available resources, relying on the built-in notification system to scan for and raise notifications.
[APP][SVC][Apr 4 2010]ReRemind V7 / Periodic audible reminders for waiting events - xda-developers
I like this one but to be fair I haven't tried the other suggestions in the thread. I'd recommend it though. Oh and by the way, its FREE!