Sadly, there's a lot of information in the engine and road sounds and tire sounds to want to cover it up with music. Maybe on some of the POS Sunday driver cars, with full interiors and muted engine noise, take a nice leisurely drive around the Nürbergring, listen to some gentle music as you cruise along, but you couldn't hear music in a race car even if you WANTED to
PD/Sony could take a long hard listen to Shift's engine and driving noises... now THERE'S the snarl and bite of a stripped out, no soundproofing race conversion. Most of GT5's car noises sound as if all the seats and carpets were still in it, even after you drop a ton of money on race modifications. The engine noise should be made MUCH stronger and livelier after the first Weight Reduction, because all that padding and noise insulation would be one of the FIRST things you got rid of... back seats, passenger seats, carpets, pretty much everything that mutes the stock car.
It's amazing how just the SOUND of a race car at full bore adds to the excitement of a game. Watch a Shift replay with the sound off, it's not much different (in its' way) to GT5, with the sound off. Turn the sound back on again, though, and Shift SOUNDS (even though it may not handle or look as good as GT5) a bunch more exciting. Can't say quite the same for GT5, though...