There used to be one called SoundJaw, but it's not so easy to find now. But SoundBender is current.
These things do not stick very strongly to the device (works with various kinds of tablets), can be easily knocked off but also easily returned, and it may be seen as a bit expensive for a bit of plastic, but for what it does I find it reasonable, although you need two if you want stereo.
AudioAmp180 is an alternative which will work with the tablet in a (thinnish) case, that's not the case (no pun intended) with SoundBender.
I find them useful if: 1: the iPad needs help given the ambient noise. 2: You don't want to disturb others too much. 3: this includes if you play music/video late at night (or in church, haha).