You can only have one active timer. In your code the timer keeps getting reset to a new number on each line.
Try this:
noteplayed = 0
ON TIMER 50
noteplayed = noteplayed + 1
SELECT CASE noteplayed
CASE 1
NOTE 50 30
CASE 2
NOTE 55 30
CASE 3
NOTE 60 30
CASE 4
NOTE 65 30
CASE 5
NOTE 70 30
CASE 6
NOTE 75 30
END SELECT
END TIMER