A gang of 9yr-olds, singing along raucously to their favourite modern beat combos playing songs from the hit parade, have declared that the Astra's sound system is 'swagalicious' and, indeed, 'sick', which I'm led to believe is a good thing. It can certainly reach ear-bleedingly loud voume levels.
Subjectively, there's more bass evident while driving (it used to go AWOL in Astra J) and speech is very clear. Whether either of those things would tick the boxes for a true audiophile, I know not.
It is also more adjustable at the bottom ed of the volume range - in Astra J I always found '1' too loud when waiting in the car, in Astra K the levels are much better regulated and can go almost whisper-quiet.