Hi Sokratis!
Thanks for helping me!
I can't reach my Audya right now but I remember clearly some Rock styles missing guitars.
I don't remember if this started from the very beginning of 4.2b installation or it happened later (because drums, bass and lowers work always). 4.1b works OK with those same rock styles (except for that mutting problem with one guitar sound, and strangely that same guitar sound plays good in 4.2b).
Perhaps 4.2b is best adopted to the coming new sounds and styles.
Best regards,
Luka