I heartily agree with Oberon. Stephen Briggs is by far and away the best reader of TP audio books. His characterisations fit my imagination of each of the characters well. Tony Robinson's voice annoys me after a while (agree with you there, Tony) and Nigel Planer makes Angua and Carrot sound as thick as two short planks and Vimes as a man with an adenoidal problem.
I do like his Granny Weatherwax though.
Admittedly the SB audiobooks aren't cheap, but perhaps they could go on a birthday wish list...? iTunes or Audible vouchers maybe...?