Right, I've had problems with sound repeatedly over the last two days.
I'm using a Creative Sound Blaster X-Fi XtremeMusic sound card with the latest drivers for Windows 7 x64.
The sound completely disappears occasionally after changing the time or location of the bus (i.e. 'dropping' the bus at a new location). No error messages are shown and the only ways to reset the sound are to reload the map or to restart the game.