is there anything you can do to the engine which doesn't count as a mod to make it sound better, for example my friend took the cover of his air filter and took the air filter out and it now make an airy intake noise

Taking the air filter out is one way to void the warranty, it is there for a reason, to stop harmful debris and build up reaching the engine.

Anything you do to increase noise is going to count as a mod though....

He took the air filter out? yeah because thats a good idea :P

mountune induction hose? will need to declare to the insurance though ...

Most ways to increase the sound revolves around the intake, so an open air filter, cold air intake, etc. is usually doing the trick. However it all counts as mods.

The only thing I can think of is to flip down the rear seats, that way you'll hear the exhaust note a bit more clearly.

Or open the windows whilst you drive, this way you can hear the turbo spooling as well as the dump valve.

Both things don't count as mods and are totally free :D