05 plate, trouble with music fading out

[Rajchumberis]

Hi
The car is just about a month old to me but is a 05 plate, 3dr 1.3. Very low mileage, so safe to assume it belonged to an old lady using within the city- that's the history, nothing modified, same as new, back then.

Now, the first time I noticed this fault was when I was driving at 70 on the motorway and noticed my music started fading away and eventually become silen, the second I slowed down to 20 driving for 5mins the music started to fade up. When the music faded out I did check to see it wasnt just the cd player and checked the radio, and it was the same. But I thought this was a one offs but on the return journey it did the same at 70mph. So I have a feeling it may something to do with the speed or the heat the engine produces at that particular speed!?

But today, I drove to the shops, less the 2miles away and noticed within a few mins of driving, music started fading out again. But the similar factor here was that it was (for our standards) really hot nearly 17degrees. We had the windows open rtc.

First question: has anyone relate experienced this?!
An second question: how did you fix it?!

I'm taking the car to Toyota this week but if it's the heat then I don't think they find the fault seeing as its lookin to be a cold day and may not heat up.

So please can someone help or shed some light on this please. This is my first car and quite gutted that there is fault with it.

Thanks in advance for your time and help.