Open the "Inspector" by pressing F2 or that little "i" in the upper left corner in the arrange window. When you look down under "Event FX" you'll find "File Tempo" (if you can't see it, it's just covered. You can grab that fader section on either the bottom or top and drag or resize it). In your case it probably says "Not set". Type in he tempo you recorded it in. Now, when you change the tempo of your song it will strectch accordingly. If this doesn't help, Also check if on the very top of the Inspector under "Tempo" you have either "Timestretch" or "Follow" selected. Choosing "Don't follow" will leave the track unaffected by tempo changes in the song.