My little AddOn. It tells you when your mana is 30% or less and plays a sound then. You can change the sound, just replace the sound.mp3 in the iMana folder .
I don't use it anymore, so it doesn't work with the newest patch anymore.
It's good now, but it would be great if you'd silence the mod for 30 seconds after playing the sound file. Since I'm a paladin, I threw Judgement of Wisdom up on the boss when the sound played (FYI: Judgement of Wisdom is an attack that can grant you 1% of mana every time you get hit). Together with Replenishment (FYI: Retri talent, 0,25% of max mana every second) and Divine Plea (FYI: 25% of max mana over 15 seconds) my mana regenerated quite fast. Of course, I do still use attacks so my mana was at 31-26-34-28-36-20 % and all the time I got the OOOO WA A A A sound . Quite annoying, so a silence would be great.