If you have an iPad/iTouch/iPhone and an AAC app already, but are concerned about the quality of the speakers in loud situations, there are some solutions. Some are less expensive than others. The best one I have seen recently is from FRS Solutions. They have the SoundPOD. There is one for the iPad and one for the iTouch. The iTouch SoundPOD can be purchased with or without an iTouch. The iPad SoundPOD is a conversion kit, so you have to have your own iPad.
SoundPOD for iPad: http://www.frs-solutions.com/products/sounpod/iPad
SoundPOD for iTouch (on their site it says iPod. Not sure if it is compatible with an iPhone): http://www.frs-solutions.com/products/soundpod/ipod