Hi Rich M !
First, great for the second question, nice that the forum can be a knowledge base sometimes

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For your first question, it quite depends on the guitar type.
If it's an acoustic one that you want to record using a mic, and if you have an iPhone, your internal mic would be just fine. If you have an iPod touch then you need an external mic, such as the Philips one. It's basically a mic with a 3.5 jack output for plugging in your headphones. If you want to plug in a PA system, you can plug it in there too BUT beware of the feedback, the sound played through your speakers will be recorded again on your next tracks, so it's highly recommended to use it with headphones only.
If it's an electric/electro-acoustic one, then you can use the PRS GuitarBud cable, which feature a 6.35 male jack to plug directly into your guitar and it has a 3.5 female jack for output too. And this time, you can plug it into a PA system without much problem because there is no possible feedback (unless you put your PA really loud).
Hope this answers your question. Feel to ask if it's not clear enough

!
Cheers,
Raphaël