You cant customize the shift sound proper. But you can for example, in a place where the ''music'' bank its not used make the shift teleport to a screen where the music played its the sound effect you want (Implying that you already know how to put custom music,tilesets,etc) since the "music" never loops.
[shamelessplug]You can also see
my level as an example[/shamelessplug]
PS: Also, neatly using Purple Pineapple's KS mod you can make a shift make the shock sound of bank 2 object 18
.