One of my dreams has come true. I got a pair of speakers from JVC LCD TV with a sound so surprising that their further purpose was made clear.
They are 3 cm in diameter and they come in miniature plastic boxes. There is a concave spheric aluminium membrane and no paper in the speaker. So I made a holder with a 2 x 5W amplifier and some grilles for the speakers and the summer alternative for the headphones is finished. The holder works as a radiator for the amplifier and the whole set can be attached or removed by turning one screw. The volume is controlled directly by the mp3 player.
You can't believe the sound until you hear it. It's really capable of producing BASS sound (from 50Hz up)and it really doesn't sound any "plastic" - it doesn't have any resonance frequency.
Of course, when riding in strong wind, i can't hear any bass sound, but it's the same with my headphones.
I normally use Koss Sporta Pro and the sound of the speakers is pretty much similar.
So I took some photos. As you can see, it's the accessory of my complex bike system described in the other thread. The hub dynamo is capable of powering those speakers completely - they don't consume the maximum power all the time.