i've fixed your Font Scroller but you have another Problem,
the CanvasGadget() and the Box() can't be moved in Z-Order into the background. and your Command: Delay (25) let flicker the box.
copy the code and run it. all is fine but if you uncomment Line 79 you will see
IncludeFile "fmodex(short).pbi"
Enumeration
  #Mainform
  #Wave
  #spectrum
  #win
EndEnumeration
Global fmodsystem.i, Channel.i
i.i
j.i
Max.i
Position.i
YCurrentWave.i
YOldWave.i
 
EP_InitFXLib()
InitSprite()
MP_Graphics3D (1024,768,0,1) 
CanvasGadget(#spectrum, 240, 150, 540, 400)
FMOD_System_Create(@fmodsystem)
FMOD_System_Init(fmodsystem, 32, #FMOD_INIT_NORMAL, 0)
File.s = "Gunther.mp3"
Dim WaveArray.f(1024)
FMOD_System_CreateStream(fmodsystem, @File, #FMOD_SOFTWARE, 0, @sound)
FMOD_System_PlaySound(fmodsystem, #FMOD_CHANNEL_FREE, sound, 0, @channel)
EP_Init2DStars(200,7,1.3,0,0,1024,768)
EP_2DStarsColor(1,RGB(255,0,0))
EP_FastRasterBar(0,RGB(40,140,250),50)
EP_FastRasterBar(1,RGB(135,135,135),50)
;////////////////////////////////////
;- SINUS FONT
;////////////////////////////////////
    EP_CatchFont32(bmpfont,?font, ?endfont)
    EP_SetScrollText(bmpfont,"  yep ! .............that's all good !! ")      
    EP_Create32Scroll(bmpfont, 0, 0,240, 0,1030)
    EP_SetScroll32Speed(bmpfont, 5)
    EP_SetSinScroll32(bmpfont, 2, 2, 30, -1)
;----MainLoop---------------------------------------------------------------
Repeat
  
  event=WindowEvent()         
  
    MP_RenderBegin() 
  
 FMOD_Channel_GetWaveData(Channel, WaveArray(), 1024, 0)
    
 StartDrawing(CanvasOutput(#spectrum))
       
  DrawingMode(#PB_2DDrawing_AllChannels) 
  Box(0, 0, 1026, 500,RGB(0, 0, 0)) 
    
    
  DrawingMode(#PB_2DDrawing_Transparent)
    For i=1 To 1024
      YOldWave=(WaveArray(i-1)+1)*250
      YCurrentWave=(WaveArray(i)+1)*250
      LineXY(i-1,YOldWave,i,YCurrentWave,RGB(0,200,50))
      LineXY(i-1,YOldWave+1,i,YCurrentWave+1,RGB(0,90,50))
      LineXY(i-1,YOldWave-1,i,YCurrentWave-1,RGB(0,110,50))
    Next 
    
    StopDrawing()
    ;Delay(25)
          
          
EP_2DStarsDraw()
EP_2DStarsDegree(b.f) ; RANDOM DEGREE
b.f - 0.1
 EP_DisplayRasterBar(0,30)
 EP_DisplayRasterBar(1,740) 
 EP_Move32Scroll(0) 
 MP_Render2D ()     
 MP_RenderSprite ()
 MP_RenderEnd()
 MP_Flip () 
 
 
 Until GetAsyncKeyState_(#VK_ESCAPE)
          
      FMOD_Channel_Stop(Channel)
      FMOD_System_Release(fmodsystem)
      End
      
 DataSection
  font:
  IncludeBinary "2.bmp"
  endfont:
EndDataSection