Also should be noted as referenced in the Twitch statement, some of those add-ons they include have their own settings, might check on that from any you have enabled. 
Posts made by VVasabi
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Opera GXRE: Opera GX makes noise randomly
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Opera GXRE: Opera GX makes noise randomly
@anesnuhanovic That is not the sound I believe I was hearing. Your sound is more of an alert type of sound where mine if I recall was more of a smartphone keyboard click. Worth the time to go through all of Opera GX's settings. There is a lot of functionality and options. If you have any extensions might start by disabling those first to see if it fixes it since I only have a few extensions and have not heard that noise yet.
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Opera GXRE: Opera GX makes noise randomly
The sound, if it is the same I am hearing, appears to come from Twitch. Can confirm this yourself by right-clicking the toolbar on the left side and un-checking Twitch, then rechecking it for that familiar pop or loading sound.
Now the fix. Click on the Twitch icon to pull up the little twitch info menu. At the top you will see a settings icon inside this menu. There you can turn off sound.