I think if you mount your phone too close to the windscreen you may well get a bit of reflection, at least that’s what I’ve found with cameras in cars. As far as sound issues go obviously if the camera is mounted in the airflow you’re going to get a lot of noise you don’t want, in the past I’ve used an external microphone placed between the seats near the tunnel and that’s got me quite a nice soundtrack. If you’re using the Gopro you need to remember to set the exposure for for what’s going on through the windscreen or you’ll find the track is overexposed.
Personally I quite like mounting the camera behind me so I can see where I changed gear etc.