I recently updated to a firewire interface and now some projects wont play. Ive found that if i select all the track outputs to none the timeline will play but as soon as even one track has an output nothing happens. Other projects play through this setup fine. What am I missing?

Try uninstalling the driver for your old interface. Then, if you open an old project that still references it, SONAR should prompt you for the new device.

+1 to that.
What I found happens if you switch interfaces then Sonar might swap the master output to SPDIF. Check your master output is correct first. 
 
Sonar also wanted to use the midi ports from the old interface. So I un-installed the old driver and it fixed the problem. 
 
You can globally assign all outputs to your master buss using the Tracks menu, look for output. 
Ctrl A to select all tracks. 

cheers guys. heading to studio soon. will hopefully report back with successfully

Make sure under Preferences > Audio > Driver Settings
that you have a playback and record timing master set.