The Apple iPhone's microphone is fairly decent as far as smartphones, but it may be not enough for your grandpa. The SoundAMP 2.0 app can turns your iPhone or iPad touch into a hearing aid. SoundAMP 2.0 is the first hearing app on the App Store, and top 5 medical app in 2010. You can download the latest SoundAMP from App Store with only $3.99.
You just need to run SoundAMP and plug in a pair of earphones for your iPhone, iPod touch users will need one with an inline microphone. The app takes sound from a mic, and amplifies it according to your demands. Just raise or lower the oversize slider until you get the volume you want.. The app stores the last 30 seconds of audio in a temporary cache, in case you missed anything the first time around.
SoundAMP improves your hearing quality in a variety of environments, such as noisy restaurants. The updated version of soundAMP fixed iOS 4.0 bug. It also supports multitasking which works on iPhone 4, 3GS and iPod touch 3G. So you can listen with soundAMP while viewing your photos which take advantage of iPhone 4 high resolution screen.