Music before the announcement is a European thing - and probably is related to the musical church/town clock tradition.
This music is apparently to 'warn' people that the clock is about to signal the hour - so that you can count the hour bells and know you haven't missed one by not paying attention to the first 1 or two bells. (I've just been in a museum in Utrech where this was stated as the reason for town/church clocks having carillons.)
This has carried over to some train/tram and bus announcements - play a little tune to 'warn' every one that an accouncement is comming.
The idea has some merit, but if you were going to do it, you do it universally, not for one type of vehical that may have some special reason for being on the network - that only gunzells care about anyway