I have inserted an audio feed directly by utilising the camera on the iPad in film mode to capture either a static image or a basic animation. This is undertaken whilst playing the required audio on any other device. This allows me to input the resultant image, including audio straight into keynote. I am able to match the timing and the style/nature of the audio perfectly to each slide. The link on the keynote slide can be resized or hidden with animation as required so as not to divert attention from the slide content.
There are a range of other option available, but not that are quite so simple or effective.
Cheers Richard Fimeri