2008 VDP N/A 120 K miles.
My long experience suggests this is one of those gremlins related to Battery health and/or good grounds: I have a regular occurrence of the "CRUISE NOT AVAILABLE" phrase in the message window but it always goes out after the first trip. I hardly even notice it anymore since it has been a regular visitor to both X350s. My newest surprise was starting the car in the morning and having no speaker output from the sound system. I was convinced it had actually died, until the second start of the day when the speakers worked fine, which is when I clocked the fact that the CNA message had been showing, but was now gone.

Does this make some kind of sense? Has anyone had radio output issues that were later solved?

 

Regarding speakers; I have not experienced it myself, just read in one of the posts, that speakers wire attach to speaker itself is prong to corrosion. ON ALL DOOR speakers. One member of the board commented that at one point all speakers went silent. I could not fathom that being a corrosion at the same time on all 4 speakers.
Never the less it was.
I can't comment on cruise control, I do not have it in mine 2004 VDP. I would think if it is battery related, then after engine started it should have consistent 12.8V or more.

 

Each wheel has a wheel speed sensor, and if one of these plays up or fails, the cruise control controller will detect it and put up a message, plus not allow cruise control to be used.

 

This would mean "ABS is not available" and vehicle stability, or whatever it is called in Jag speak, is not available.
DISCLAMER: this is my guess(logical deduction) from understanding how vehicle systems works...
Alimentary, Watson!

 

Don't be surprised if you find multiple speakers that are dead for the reason Big Koshka posted above! I had 4 dead ones that I repaired with the solder trick on the wire. Strange but reported many times for this model of car.

Is it possible you might have a bad clock spring under the steering wheel? Just a guess.
Can you check for codes to see if the ABS sensors are the problem as posted above?
Do you have adaptive cruise or regular cruise control?
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I had "cruise not available" on first startup of the day many times, and usually along with "cannot set parkbrake." (It wasn't "cannot release parkbrake" because with the event happening so often I got in the habit of preventing the parkbrake getting stuck on by pressing the parkbrake button down when removing the key.)

New battery fixed all of it. Didn't bother having the battery tested, I'd just read here about the car's sensitivity to voltages being low by even a tenth of a volt or two, and it never happened if I left the car on a trickle charger that was usually on my motorcycle. That last part absolutely confirmed weak battery for me. Hasn't repeated in nearly two years now.

 

To clarify, my cruise is regular, not adaptive, and my radio output that morning was total silence. There must be more than 4 speakers in the car, but zero were responding to normal screen input or button pushing from me. I thought I had inadvertantly discovered a "mute" switch. Those of you who have had the door wire issue, was your silence global? Hard to figure that one, but I will search the forum.

Thank you all.

 

There is a mute button, but the display will show Mute On, at least on my non-touch-screen system. Top-left thumb button on the steering wheel mutes audio to make a phone call.

The speaker wire issue is not global, it affects that speaker only. Depending on whether you have the Premium or the Audiophile system, you have either 8 or 12 speakers in the car. Both system have the same four door speakers, the ones affected by the corrosion. Premium has a main speaker and a tweeter in each door. Audiophile adds a pair of "subs" on the rear shelf and a pair of ambience speakers in the dash.