I'm working with Sibelius 7 and even though I'm adding 8va lines at some parts of my score, when playing it back I get the same sound I should be getting if I didn't have them.
Any ideas why this could be happening?
I just tried it (version 7.1.3) and it played back an octave higher. I selected Notations, Lines, Octave Lines, 8va, then clicked on the note where it started. You can drag the bar out as far as you want the effect. After it's there, you can click on the 8va, then press control-shift-i to make sure it's set for playback.
If you still have trouble, there's a good Sibelius forum that could probably help: