OP - make sure you have the AF turned on, this is 'Alternate Frequency', it should auto tune the radio allowing for geographical shift in the frequency of your nearest transmission tower. As for the buzz on speech, this could be due to the radio not following the frequency, FM or Frequency Modulated transmissions get furry round the edges.
Thanks Andy, I will have a look at the AF angle. The dealership tuned the radio so a delve into the manual may be required. As far as regional frequencies are concerned, this is happening during local driving so that shouldn't come into it but if the frequency is drifting because AF isn't on then that could be it!