Walking through a parking lot and someone takes out their fob and somewhere a car responds ''beep, beep". Another fob and “beep, beep, beep”.
It occurred to me these people might want individual recognizable downloadable ring tones like cell phones. A new idea for inventors? Then I thought this must already been offered, hasn’t it?
Some people would certainly like it as a novelty. Some factors which limit the utility:
The range of the car remote is pretty short. Most of the time, the car is in sight of the person - the beep and the flash of the car’s lights merely confirm that it responded.
You might think that the beep would be useful if you don’t remember where you parked. But a lot of remotes may be like mine, which only beeps when it locks, not when it unlocks.
There is not constant beeping in a parking lot, and different models do beep differently. So there is no difficulty in detecting your car responding to your button push.
Somehow, I was reminded of this. A few years ago, officials in Shanghai banned the use of car horns to combat noise pollution. The result? A booming business in car horn replacements that spoke, or played music:
http://www.mobilemag.com/2007/06/29/shanghai-bans-car-horns-drivers-switch-to-music/
I have mine set to only flash the lights only when it locks. I have been parked in open pitch black fields with hundreds of cars trying to find my car. The simple solution is to just push lock until you find it. Then unlock it. 
Duh. Yeah, I’m not awake this morning.
Also, with a key fob, you know exactly when you’re pressing the button, and can hear exactly when it beeps. Someone else beeping their fob even a few seconds before or after yours won’t confuse you. With a phone, though, you usually don’t have any idea at all when someone’s going to call you.
I would love an option like this. My car doesn’t beep when I fob the lock (boy that sounds dirty). Instead, it honks the horn. Way too loud! It’d be cool if I could set it to play some Foreigner. You’re as cold as ice!