Hello Anna,
I don't know if this will be of any help,
Years ago I fitted an old car radio and a pair speakers into a specially made portable wooden box for use in my garage. It works a treat and to my ears sounds better than many commercial sets. I also fitted a 12v power supply into the box, but just making up a 12V lead would suffice.
The same could be done to-day with the car radios with CD's. It rather depends on your DIY skills.
Alternatively, with relatively simple wiring it would be possible to arrange a speaker socket rather than a 12V socket, and you simply plug in a pair of speakers
Third option, - there are many portable radio /cd players on the market, just use the internal batteries.
Some use a plug in power supply, which may be 12V - simply replace the power lead.
If the radio needs less than 12V there are many adaptors available to reduce 12V to a lower suitable voltage (Maplin Electronics)
So I think there are many way you can enjoy your music in the awning, but do spare a thought for other site users with the volume.