as some of you already noticed, the levels now play sound
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In fact, they were always meant to play sound, using the <bgsound> HTML element. This was problematic because it's a non-standard element invented by Microsoft which was only supported by Internet Explorer. Firefox was able to play bgsound using a plugin, but this was all very unsatisfying. Some people probably didn't know or didn't care enough and played notpron without sound.
I went through all the levels and replaced all <bgsound> elements by the cross-compatible, modern HTML5 element <audio>. Now you can play notpron with sound in any modern browser, without plugins!
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I'm stuck in this level for hours and the sound is driving me crazy!
You can always mute a tab by clicking the sound icon in it:
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Chrome users have it harder - apparently, clicking that icon in Chrome does not work out-of-the-box, you have to right-click the tab and select "mute tab" instead. If you want to be able to click the speaker, you can actually make Chrome do that:
Copy paste this address into your URL bar chrome://flags/#enable-tab-audio-muting, press Enter and click "Enable", then restart Chrome.
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Enjoy!
Jay