Chrome's audio detection is quite smart if I'm not mistaken. A tiny amount of sound is usually seen as 'silent' too, since it'd be practically inaudible.
One of the first comments on the link above mentions this as a potential exploit; they said they would watch for people attempting it and attempt to mitigate it if it becomes popular.
Background tabs usually do not have a visible impact for user (tabs with audio are always considered foregrounded).