If they gave people the freedom to do that, many would probably create strange fantasy tracks.
That wouldn't really go with the title "Real Driving Simulator".

So, I'm having doubts. However, I'd still like to welcome a feature like that (but maybe with some limitations). It shouldn't allow impossible turns and stuff.

it may be if they were indeed talkin  about the ability of the gamer to edit the race track's 

Sony has been on the : play ,create,share design for this generation

so the ability for the gamer to edit the track in GT5 would indeed go along with that theme. but until their is further conformation

on this , IT's not known if its about music or its about the race track's

You may be right, we don't know what type of tracks it is. But I ask a simple question, what sense would it make for you to be able to edit the audio tracks only in a racing game? :P

because right now that is the only way Sony can do custom audio tracks:

its because of Legality:

Microsoft patented the Idea of "IN GAME CUSTOM SOUNDTRACK'S" before the PS3 launched.

To be honest, the amount of time they've had to develop GT5, i won't be surprised if it IS true. If it is however, that's going to be amazing. If not, i could understand. For a simulation racer, the physics could get wierd if given the ablitly to make your own tracks.