In Bb 2.0 is a collaborative music and spoken word project conceived by Darren Solomon from Science for Girls, and developed with contributions from users. 20 different YouTube videos embed in the same page, each with one instrument playing a tune, but where harmony is very well achieved. The videos can be played simultaneously - the soundtracks will work together, and the mix can be adjusted with the individual volume sliders. And you can combine any choice of instruments and play a different music each time, worth a try.