[nekopa]

I don't know if its still available for iPad (I have the 1st iPad) but I used to use an app called AudioNote (I think).

With this app you could record audio, and any notes you took are synchronised to this audio. It was super useful...

[thirdsun]

Apps like Notability [0] do this very well these days. You can combine text, pen and audio input, though I'm not sure if the audio is synchronized.

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[0] http://gingerlabs.com/