Unfortunately, the sound on the 5550 is not the loudest, i dont think there is much you can do - check HP site incase they have a sound driver like they had for the 5450!
A couple of things that might help u:
Arkon Audio Adapter - amplifies sound to your car Radio through a FM signal - http://www.totalpda.co.uk/Arkon-SF25...o-Adapter.aspx
Or, Arkon Amplified Mount - http://www.totalpda.co.uk/product4576_32.aspx