Lexus Reading are open till 6pm, so I just spoke to the service department.
He told me that electrics, including the stereo and speaker system were covered by the warranty.
Although they were a little unclear as to the service requirements for eligibility of the warranty. The guy told me that it needed a full Lexus history, whereas the other dealer I have used, Jemca Edgware Road, said that the previous history was irrelevant as long as you get it serviced at the dealer on time during the warranty period.
Having told him this, he asked his manager who said that as long as the car had been serviced, it didn't have to be at Lexus beforehand.
So there seems to be a little confusion on some aspects.