This is an example it has super Mario galaxy 1 and 2 ost 

1-Go to this link : https://archive.org/details/SuperMarioGalaxy2Ost for Mario galaxy 2  or https://archive.org/details/SuperMarioGalaxyOST  for Mario galaxy 1

2-find the download options

3-find an option named ogg vorbis, then click on it and you will find a list of ost

4- click on any ost  and then copy the URL , for example : https://ia800701.us.archive.org/12/items/SuperMarioGalaxy2Ost/BowsersGalaxyGener... 

5- go to the editor and then click on the setting of the area == click on any song name and delete it all and then past the url in it and then click Enter(in the keyboard)

6-test the level and wait a little bit if the music doesnt change or start quit the test and then go to the  area setting  again and wait

this is how i get custom music hope this help you :)

For the doors :1-teleport in the same doors in the area (my explain might be bad) 

the doors that has the same destination tag will teleport to each other

for doors in different areas, in the areas you can see each area id 

1-make sure the door is remote if you want it  to teleport to another area and then write the area id in the area id in the door setting