How do I make a different song play when you ride a vehicle?

That will require a little plotscripting. If you havent plotscripted before, start by reading the Plotscript Tutorial first.

The trick is to use the vehicle's mount script and dismount script. Here are some example scripts

plotscript,start vehicle music,begin
  play song(song:vehicle music)
end  

plotscript,stop vehicle music,begin
  play song(song:map music)
end  

You must adjust the names of the songs to match the names of your actual songs as they are defined in your .HSI file