You can hear sound effects when you move your mouse inside non-Hearthstone window if you have enabled sound in background (mose over events are recognized when the window is not in focus)

[culinko]

If you enable the "Sound in Background" option and switch to other programs such that their windows are overlaid over the Hearthstone window in the background, when you move your mouse inside the non-Hearthstone window over the locations, such as your cards at mulligan or the history sidebar tile, you can still hear the sound effects as though the active non-Hearthstone program window is not present.

• http://us.battle.net/hearthstone/en/forum/topic/12033553477
• https://www.youtube.com/watch?v=WDWvCeESM84

The sound of the UI buttons was fixed in Patch 4.2 or 4.3, however other sound effects, such as your cards at mulligan or the history sidebar tile still make sound effects as if Hearthstone window wasn't in background.

[culinko]

You can hear UI buttons again in 5.2.

[troggnostupidhs]

Possibly fixed

https://playhearthstone.com/en-us/blog/21098848

Hearthstone’s engine has been updated to Unity version 5.6.

[culinko]

You can no longer hear the UI buttons (this was already fixed once), but you can still hear the sounds when hovering over your cards at mulligan or over the history sidebar.

[troggnostupidhs]

~This should be fixed.~

edit: I tested wrong. 'Sound In Background' should be on.

[culinko]

I tested this just now and it's still present for me. I am able to hear sounds when I hover over my cards at mulligan and over the history sidebar entries. I'm running hs in windowed mode and tested it with regular Notepad in front of the hs window (also happens with Google Chrome).