In a way I think you have this backwards. A bottle clinking, in a club, becomes a part of the surroundings, while you are there. In other words, you almost expect to hear it, so it is not startling after the first one or or two. On the other hand, conversations directly behind you, are more likely to get your attention, since you cannot see where it is coming from, as opposed to conversations in front of you. Of course my opinion is based on my paranoia of a tiger, or something else, stalking me.
We unconsciously filter out much of the conversations behind us, until they cross a threshold where we become consciously aware of them. We can consciously ignore them until they cross another threshold where those sounds are competing with what we are actively concentrating on. We do the same for reflections, reverberation, and whatever. When a bottle clinks, it crosses those unconscious thresholds which other chatter does not - we're well evolved to unconsciously react to a twig-snap in the woods so that we direct our consciousness towards the tiger stalking us.
You're talking about conscious processes and the conscious direction of hearing. The loin's share of human hearing processing is done unconsciously.
Its useful to contrast hearing
, which concerns the mechanical and unconscious mental auditory processing systems which are not consciously directed, with listening
, which is a higher order function dependent on the foundation of hearing, yet concerns consciously direction of attention and concentration on content).
A bottle clinking, in a club, becomes a part of the surroundings, while you are there. In other words, you almost expect to hear it, so it is not startling after the first one or or two.
^That's conscious listening overriding unconscious hearing. We're evolved to take unconscious notice of things like bottle clinks and twig snaps, until we consciously or semi-consciously categorize the source and disregard it as a threat. Then we may begin to consciously ignore it as we focus our listening attention elsewhere.
When a conversation behind you is loud enough to be heard, it has crossed the first threshold of hearing. You can and do consciously ignore conversations behind you until they begin to cross the second threshold where they become too distracting to ignore, and that's when they start to actively interfering with your listening.
Your paranoia about tigers is a high-order conscious / semi-conscious thing and doesn't really have anything to do with evolutionary hearing traits. This is not about paranoia. Reaction to surprise transients (the twig snap, the bottle clink) is unconscious and an evolutionary derived over eons by selection of those who startled and ran away rather than not-starling and getting eaten before they could reproduce. Complex animals have a startle reflexes rooted in self-preservation, and this is a well known auditory specific aspect of that.
Of course if you are paranoid concerning tiger attacks, and you suspect that you are danger of one, you'll consciously direct your hearing process to listen for twig-snaps. Which is sort of the opposite of hearing bottle clinks, categorizing them as non-threatening and not of particular interest, and subsequently ignoring them (until they cross the second threshold).