The usual radiating sources are the internal clocks. These can be anywhere, but there are advantages in having them in the 10 to 25MHz region - cost of crystals, power consumption. They produce harmonics at multiples of the clock frequency, and get divided down to lower frequencies, which also get radiated, and the lower frequencies can get into NDB receivers. The higher frequencies get modulated by the lower ones, so it's not a nice clean frequency, but a whole slew of them. In order to met the few standards there are, one trick is to 'dither' the frequencies, which effectively spreads them out but raises the whole noise level. An analogy is having a pile of manure in one corner of a field that stinks out that area as opposed to spreading it over the whole field and stinking out the larger area at lower intensity.....
You may well find that some MP3 players don't emit, and others do. That makes it very hard to draw a general conclusion. Add to that the susceptability of aircraft systems, especially as aircraft age and the effectiveness of ground connections on shields and so on decreases, and abrasion gets at cables and so on.
The CAA comments to me were related only to hearing aids operating in the 800 -900 MHz region, with output powers below 10 nanowatts, by the way.