UPDATE: The LED flicker has stopped. Only took a couple of days. I don't know if this is the car 'getting used' to the different bulb or something, but now the lights are on instantly without any flickering at at all. Very happy with them.
Also, the audio is well and truly bedded in now so I'll post a proper update on that when I have a few more minutes to write it down. In a nutshell: it's ace.