Perhaps something like this ...
Code:
Freq        var word
say         var byte
countera    var byte
PlayButton  var PORTB.7
SoundOut    var PORTA.0

loop:
    If PlayButton = 0 THEN 
      For countera = 0 to 70                ; Siren UP
          Gosub PlayTone
          if PlayButton = 1 then ButtonUp
      Next countera 

      For countera = 69 to 1 step -1        ; Siren Down
          Gosub PlayTone
          if PlayButton = 1 then ButtonUp
      Next countera 
    endif
 ButtonUp:
    Pause 50  
goto loop 

PlayTone:
    LOOKUP2 CounterA,[902,902,902,884,884,884,851,851,835,820,820,806,806,_
    792,778,765,752,740,740,716,716,705,705,683,683,653,663,663,644,635,_
    618,627,602,594,578,578,557,564,549,531,531,519,513,501,492,475,465,_
    465,456,442,434,422,414,406,389,389,373,373,352,334,329,318,307,296,_
    283,279,263,252,249,238,232],Freq

    For say=1 to 25
      High SoundOut
      PauseUS Freq
      Low SoundOut
      PauseUS Freq
    Next say
Return