mbox, ibook, protools- sounds works intermittently

[vicsclipse]

First my set up. I have a first gen mbox with protools 6.4le. I'm running it on an ibook with 10.3.9 with the quicktime fix (reverted to 6.5.2). I thought that might solve my problem but it doesn't. So now to the problem. I'm a novice so I apologize ahead of time... unfortunately I updated my main computer to 10.4 I haven't been able to use my protools for a very long time so I suck at it.

So here's the story. I can load up protools fine, start a new session and as long as for i/o settings I pick stereo mix I can get everything working. I recorded a couple tracks and everything worked just fine I messed around with them and then after saving the session I quit. I shut down the computer. I then came back an hour later and wanted to fiddle with the song a bit so I opened protools, went to open session and opened the project. Opens fine, I see all my audio is there. I click play... and I don't hear anything. I can still monitor directly from the mbox, but I can't monitor anything that is already recorded. It shows that it's being played in the mixer and such. The level indicators are moving but I can't hear it.

I'm sure it's something silly... but I can't figure it out. And I have no idea why it works when I do a new session but when I load a previous session I can't hear it. I particularly don't understand why anything would change. Any help would be greatly appreciated and I apologize if it's a silly question but I can't figure it out for the life of me! Thanks ahead of time!