Added sample files

Author: cwoffenden

test/webaudio/audio_files/README.md

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+Emscripten Beat and Emscripten Bass by [CoLD SToRAGE](https://www.coldstorage.org.uk) (Tim Wright).
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+Released under the [Creative Commons Zero (CC0)](https://creativecommons.org/publicdomain/zero/1.0/) Public Domain Dedication.
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+To the extent possible under law, OGP Phonogramatica has waived all copyright and related or neighbouring rights to these works.