To quote Prodigy:
F THEM AND THEIR LAW!
Distributing a track with a music file is illegal (playing it is not). However few bands have the backing/funds to pursue a lawsuit, and even if this is a criminal offence in some jurisdictions, they have to A. learn about it first and B. care. And it seems governments these days have
slight budget issues. It's like jaywalking - illegal, but unless you do it in front of a cop nobody will care enough to report you. And even if you
do do it in front of a cop, he may well have better things to do than stop and give you a ticket.
Oh, and what is much worse than sharing music is sharing
imported cars via Carpark. That site is legally on unstable ground because it is full of imports from other commercial games, and while the car "maker" has no intention to profit, Nadeo does (and they themselves encouraged the use of custom cars in an interview). Even if you only make skins you're still guilty because you need to include the car model file as well.
My recommendation is to apply good faith and use tracks by unknown artists and credit them in the intro. They may even sell an extra cd, and your track will stand out more if you pick an original music genre. A bad choice would be an obscure techno remix - there's only one of it, so people will not buy a cd and it may be nontrivial to find online, incentivising people to try and record it.