You are Pro Tools 9 standard right? Not HD?
You have likely been bitten by Input Monitoring/Auto Input Monitoring. This often happens by folks accidentally pressing the Command-K (ALT-K on Windows) keyboard shortcut that toggles modes.
The Pro Tools reference guide describes input monitoring very well. Make sure you look in a Pro Tools 9 version, not a recent version (track input monitoring feature--which is fancier than what is happening here--was only in Pro Tools HD in the Pro Tools 9 era and is now in Pro Tools as well as Pro Tools Ultimate versions and is more flexible/powerful). BTW track input monitoring which was added to Pro Tools (standard) in an early version of Pro Tools 12 can be a good reason for folks to upgrade to more recent Pro Tools ... more for folks running larger recoding sessions, but once you use it you never want to go back.
Or the CliffsNotes version:
https://www.sweetwater.com/sweetcare...t-monitor-only