(sorry, some more for you to look over,,, Got a little carried away today and completed all the A's in my standards garage. Route X can't get here soon enough to distract me!!)
Aston Martin DB9 '03 - Stock
Stock exhaust is really good but a little quiet, Sports has a lack of definition , almost sounds like a 4cyl, Semi race is great at idle, but sounds more like an outboard motor, Racing exhaust is a little bit too much like the Cadillac Cien.
Aston Martin V8 Vantage - Semi Racing
Stock has a nice European V8 sound to it, Sports has a rather strange note, like the Lancia Delta with full race exhaust, Semi race has a note very similar to the Corvette with a nice range of notes, Race also sounds like a speedboat.
Aston Martin Vanquish - Stock or Semi Racing
Stock exhaust has a very nice sound after 4000rpm, Sports has a very similar problem to the Vantage, sounding like a Lancia Delta up to 5000rpm, Semi racing has an extrememly nice tone, almost like a lamborghini, Race exhaust has an extremely aggressive tone, like a TVR Speed 12, but sounds like a V8.
Audi A3 3.2 Quattro '03 - Stock / Sports (All sound good though)
Stock has a decent European V6 tone, Sports has a really nice lower region and sounds awesome up to 5000rpm, but gets a little raspy after this, Semi Race sounds like the Supra RZ, the Race exhaust sounds like a Skyline, both out of place but very nice.
Audi Le Mans Quattro - (None, they don't have the V12 note the car should)
Sports is good up to 4500rpm, but sounds like a V8 after this point, Semi race loses all definition and also sounds like a V8, Race exhaust also sounds like a V8, but a nice European tone.
Audi Nuvolari Quattro - Stock (or none, again)
Sports is good between 3000 and 4500rpm, but not so outside this range, Semi race is just bad, Race sounds like a tuned European V8.
Audi Pikes Peak Quattro - Stock / Sports
Sports has a nice, low-slung V8 sound, Semi race has a good open-throated sound with immense low rpm notes, Race exhaust reverts to the generalised European V8 sound again.
Audi RS4 '01 - Semi Racing
Stock has a generic V6 note, Sports is a more beefy sound, but still very generic, Semi Racing has a really nice deep-throated warbly six cylinder sound, Race exhaust is nice, but a little too similar to a Skyline.
Audi RS6 '02 - Semi / Titanium Racing Exhaust
Stock has a very nice warbly V8 tone, Sports is great at idle but is rather bland in the revs, Semi race has a great European V8 sound, Race has a slightly more aggressive tone to the Semi Racing one.
Audi RS6 Avant '02 - Stock
Stock has an awesome warbly V8 2 toned sound, Sports has a lack of definition and character, Semi is like a badly tuned Euro V8m Race is the same generic Euro V8 tone as pretty much most of the German V8's.
Audi S4 '04 - Semi / None
Stock is a very dull V8 sound, Sports has a really nice idle sound, but no definition in the revs, Semi race has an aggressive European V8 sound, Race has a total lack of any distinctive sound.
Audi S4 '98 - Semi Racing
Stock is a little dull, although has V6 notes to it, Sports is slightly more beefy but is basically the same as stock, Semi Race has an impressively nice warbly V6 note, close to the Alfa Romeo, Race exhaust is a little bit like the NSX.
Audi TT 1.8T - Titanium Racing Exhaust
Stock is very good for a 4cyl, but a little quiet, Sports is just a complete mess, V6 midtones, S2000 high-end, Semi race has a complete lack of definition to it, Race exhaust has a really nice lumpy idle and a unique overall note.
Autobacs Garaiya - Stock / Titanium Racing Exhaust
Stock has a reasonable sound, although sounds like a smaller engine than 2L, Sports has no definition, and almost sounds like an S2000, Semi race also sounds like a high-revving Honda, Race Exhaust is very generalised in sound but the best of the bunch, in my opinion.