Just guessing, but in non-rhotic accents like RP and New Zealand English, the R in car is not pronounced unless the following word begins with a vowel. Any other ideas, maybe from an American English speaker?

For most American dialects, the sounds are nearly identical, but not quite: at the beginning of a syllable, the "r" is an approximant (a type of consonant that starts with an emphasized vowel sound), rather like "y" or "w", whereas at the end it is a true vowel, again like "ee" or "oo" respectively.

I'm a native en-gb speaker: I'd say the R in red is hard (ruh! like huh!) and the one in car is soft (aspirated; "ar") but I don't know IPA so couldn't really give you any more than this.

Reh-ed and ca-arrrr//the first is a soft sound, the second says itself.

With my New Zealand accent, no matter how hard I try I can't make the "R" in red and car the same! Car for me is pronounced something like "Kah".

In New Zealand English, you should be comparing the pronunciation of R in “red” and “car audio”, to ensure that the R is actually pronounced at all. Some accents don’t even pronounce it in the latter. (There’s a good one for the spelling reformers.)

And be careful with “ah”. I have a strong suspicion that we pronounce it very differently in New Zealand compared to North America.

With my Australian accent I wouldn't pronounce an "r" sound in "car audio" either -- just switch straight from the "ahhh" sound of "car" to the "orrrr" sound of "audio".

Speaking of which, it wasn't until quite recently that I realised not all accents have so much trouble with the ambiguity of the Ford Ka.