"HDD:Users:<username>:Music:<mp3folder>:" returning p
repeat with i in list folder (alias p)
if i ends with ".mp3" then
set the name of card button 1 to i
end if
end repeat
みたいな感じでしょうか。
File を開くのは
on mouseUp
tell application "QuickTime Player"
open file (p & the name of card button 1)
end tell
end mouseUp