Re: add audio-comments/updates/street names
Well, my bikes all have mirrors, and they aren't actually rare any more.
They're not always as obvious as mine either, indeed there are some which fit inside cycling glasses...
The noise of a vehicle is almost always useless - yes you can tell whether they've put a baked bean can on their exhaust - but you can tell that from one ear as easily as two.
I don't think I've ever had to make any sort of manoeuvre based on hearing a vehicle - and I have been cycle commuting for about 9 years now. I don't wear earphones normally, but have been on the odd conference call whilst cycling home, and one earbud in makes absolutely no odds to my awareness of other road users - a buff is useful to keep the earbud in whilst cycling though, and reduces wind noise (which is a much bigger issue for some cyclists than you think - a 25mph wind over your ears makes it fairly hard to hear anything).
I have cringed at least once - hearing the squeal of tyres and a car in a clear loss of control line astern is an unpleasant sound - but there was nothing I could reasonably do about their problem (I put some more power in, and they stopped (in a cloud of blue smoke) before hitting me).
When I'm in stop start traffic I'm normally watching for the attention of the driver of the vehicle I'm about to pass - you know making sure they're not about to just randomly pull right without indicating, other things like that. I'm not exclusively staring at their mirror, but it has the majority of my attention.
Motorists tend to fall into two camps: Those who have seen me coming from a way back and pull left as I approach, and those who wouldn't see a fire engine.
From my ears (which are reasonably good, as attested by an audiologist, and I do occasional tests on higher frequency stuff) I can't tell which lane a vehicle is in - I can tell that I want to look in my mirror, or make eye contact with the driver. I also can't do anything with the information - so what there's a car coming up behind me - I can't teleport (or I'd have teleported to somewhere prettier to do my riding). My only reasonable choice is to continue doing what I'm doing.
In a city environment this is even more so - there is so much traffic noise coming from all around, and bouncing off so many buildings, that there is almost no information left in it. There are also lots of close junctions, and an audio queue that tells you what road name you want makes far better use of one ear's attention than "there's still traffic around"