Code:
--Note this only works if the target Folder:File is in that same Folder as the script
set holdScriptNameLength to 9 -- this is the length of your scriptname including .scpt
set myPath to (path to me) as string
set myLength to the number of characters of myPath
set myPath to (characters 1 thru (myLength - holdScriptNameLength) of myPath) as string
set myPath to myPath & "Inside:song.rtf" -- folder : folder : fileName.extention
tell application "Finder"
open myPath
end tell
Any Path manipulation in AppleScript is not nice, thats why people use things like Perl for jobs like this.