I haven't heard of it being classed as such unless it was a high end machine but I wouldn't give them the chance to deny a claim over it. If it's in the terms of your policy let them know.
A modification is any change you make to a car after it has left the manufacturers. Therefore an aftermarket stereo is a modification. It's the same if you fit a tow bar on after it's been bought. it'll still be classed as a mod!