I waanted to point this out before my net got cut off a week early....
The issue with audio cues are in no order
1. Different camera angles have different sound levels, a cockpit view has a different sound perspective than a chase or hood view,for instance.
2. Also each car engine is different, compare the engine in a Ford Focus to a Dallara Indycar in terms of loudness and type, the Dallara is a lot ouder and harsher, for instance, multiply this byhaving other cars around you and you run into isssues.
3. You don't want audio cues to tell you to turn in if there's a car coming up your inside. Codemasters F1 games have a visual cue, a little arrow on the bottom of the screen to tell you when cars are nearby, Euchtechnyx's NASCAR games had a top down radar and a spotter to help you figure if guys were coming up behind you, etc, but for audio cues I'm not sure how it'd be done
Lastly...you want the audio cues to be noticeable enough but not so loud they are drowning out everything else, e.g. the engines or tire squeal or ambient sounds or music. You don't f.ex. want a beep cue to turn in and take out an opponent because you didn't know they were there, because the beep drowned out their engine sounds.
There is however a real life solution. Look to the USA and oval racing....in that discipline drivers have one, at times two or three spotters. On ovals they are on top of the highest stand and tell the driver effectively okay, car coming on your outside. The games that include spotters cheat a bit by haing the spotter know exactly where the appproaching car is....to a point.
NR2003 has it so if the car is within X distance on your left/right, usually your rear wheels in the base cars (it varies with modded car models but they have the same physics hitbox), it'll trigger the spotter to tell you car outside, but it won't tell you if the line to your left is making a run. I had full sight and still got dropkicked halfway to Canada by the spotter not telling me an opponent had stuck their nose at my rear wheels. I spun that guy out as payback under yellow.
So yes, you could have a spotter system, it could in theory work but the proble, and I know this from former NR2003 devs I know in the inudstry was they ran into issues of in their builds they had the spotter trigger for cars in the pits or on the track if you were on pit road, and in one case, for a rolling start all you heard was car behind, car behind, car behind, nothing about car outside....in fact I can't think of a released game with a spotter that tells you if you got a car approaching except iRacing, and that's built off of NR2003 code and they aren't likely to share any of their stuff.
So what other options?
Community spotter packs? They could work.
Have a friend with a good set of eyes spectate the session and have his view on your car, be on SKype or something and have a real live spotter in your ear? Also doable. I've done it before for PCARS races and rF1 races where the deafault spotter sucks. It also opens you up to making new friends