fix Issue #1038 - sound.seek()

this is to fix issue: https://github.com/kivy/kivy/issues/1038
got the tip from: https://groups.google.com/forum/#!msg/kivy-dev/prKCdsBW1CA/ojgBH5afK1IJ

I tested it on 1.6 and works.
This is my first contribution, so double check!