On GotSound.org we are using wavesurfer.js to play sounds in a browser.
It's works only in modern browsers supporting Web Audio.
MediaElement fallback to support old browsers like Internet Explorer not always works.
You can find more information about browser supported Web Audio API here.
Please, consider to upgrade your browes to one of the following:
Inside a huge power station. Made with Nlog PolySynth and B-step sequencer, recorded with Audio HiJack, edited with WavePad, all on a Mac Pro.