Do academic peer reviewers receive any salary? If yes, what is the annual average?
The vast majority not a cent, of whatever currency (see this answer for exceptions).
Peer reviewers are asked to volunteer, and they volunteer because they know that someone else would have to review their papers too (as Dilworth points out in a comment there can be other motivations too).
Said differently, peer reviewing is a mutual exchange, necessary for science to work (though the peer review system has been sometimes criticized).
I think not for journals, but sometimes "token" amounts, really "honorariums", for national-level grants. I've been asked to review/referee some grant proposals from some small countries, with perhaps a few hundred USD offered as token. I don't view it as any serious sort of "salary", certainly not enough to induce me to do a thing I didn't want to do, and I don't imagine the people offering it would think so, either. Rather, I suppose one could have a cultural viewpoint in which making some token offer is a good thing. I can equally-well imagine that making a too-small offer could be construed as an insult... ?!? But much public discourse is sooo insulting already that I don't usually think in those terms. :)
It is true that most reviewers (actually, nearly all) do not get paid. It is said to be an altruistic act that would be considered to be prestigious. But, there is a journal that functions differently. It is called Collabra, you could read the news about it here. There are others who do this feature too.
There are several concerns on why this should be paid as reviews do devote their precious time for this. The fact that they continue to do so for free is what still makes the publishers feel it is fine not to pay for them. As long as altruistic reviewers (well, in the case of money) exist, journals will continue to function without paying the reviewers.
This is nearly the same question: Are there any journals that pay reviewers?
The accepted answer that post would list you quite a few journals who pay their reviewers.
If you are concerned about whether reviewers should get paid for this, there are a lot of deep discussions about this issue you might want to take a look at:
In my limited experience (mathematics).
Referees for papers submitted to journals or conferences: no.
Books, perhaps. On one occasion, as reward for reviewing a book proposed to be published, I received a "free book" of my choice from that publisher. On some other occasions, in return for writing for publication a review of a book already published, I received a free copy of the book.
Once for reviewing some proposed mathematical software I got a free copy of it.
Referee of candidates for promotion or tenure: mathematics, no; computer science, yes.
No, reviewers are not usually paid by the journal. However, there seems to be a company, Rubriq, which offers paid peer-review services. I don't know who their current clients are, if any.
A couple of years ago Scientific Reports initiated a project where authors could pay for fast-track peer review, guaranteed within two weeks through Rubriq. The editorial board revolted over the decision and threatened to quit. They argued that doing this would create a peer-review market driven by profit, mostly with undesirable effects on the quality of reviews (reviewers are not selected based on their expertise), and that it would create a two-tier system where researchers who can afford this service would have an unfair advantage in publishing. Finally the project was cancelled.
Take a look at the following webpages:
Concern raised over payment for fast-track peer review (Note: Nature is not exactly a neutral source for this)
http://www.peerreviewneutrality.org/ is a site created by the editors.
This story illustrates why most researchers would oppose a system where peer-review is paid. There's also the issue of nearly 100-fold greater salaries on some countries than in others, so what would be a reasonable amount to pay for a review? In the end, paid review could likely create incentives which are not at all aligned with the quality of research.