Well, this is probably not your answer at all but I had something similar happen today at the broadcast studio with 8.5 no less.  In a spot we produced yesterday which was just one track of voiceover, one track of SFX and one track of music...the music and voiceover wouldn't play when pulling it up this morning.  Tried a bunch of things and looked for a bunch of things.  No anwers and no love.  So I just copy/pasted those two tracks to two unused tracks.  Everything fine again.  I'm betting your particular tracks were routed to a buss (or busses) that were off.

Ok posting from a phone... Realize that my previous post is stupid now. I meant to say: I will watch this thread closely

"yorolpal"
Tried a bunch of things and looked for a bunch of things. No anwers and no love. So I just copy/pasted those two tracks to two unused tracks. Everything fine again.

Forgot to say that... if I had used your workaround originally when faced with the problem, it would of fixed it. So I have to give you an A+ for finding an alternate solution.

Pure audio as in, audio waves (acoustic guitar) and blank output tracks of a sampler (Superior Drummer 2). 

I have, on occasion, deleted envelopes, on tracks and then saved the project not realizing that when reopened those tracks volume sliders might be (and have been) set to zero.  Which can cause temporary panic. 

Ah...I need to actually read the whole thread.  Yup, I've had this kind of thing crop up in X1 quite often (but today in 8.5 was a first) where some tracks just won't play.  So I'm keen on finding a solution...which Alegria seems to imply exists.  Good brain teaser and good exercise.

Here is another stab in the dark from my previous dealings with this issue.  Sometimes it is due to some plugin I have in my bin's on or off state.  Now I'll go read the whole thread, thank you.

"ohgrant"
You didn't press the History eraser button did you?

The big shiny red button? errrr... no?

"yorolpal"
Here is another stab in the dark from my previous dealings with this issue. Sometimes it is due to some plugin I have in my bin's on or off state.

Your stabs in the dark are putting you in a real hot place. 

Well...like I say...I've been there...done that...but now I can't remember what?  It's hell gettin old, ol pal.

It's a plugin, JT.  Or plugins.  Or something in or out or on or off in the FX BIN.  And it's not the faulty state or settings on one or more plugs Alegria, ol pal??

This is just like reliving any given Sunday.  Troubleshooting instead of making tracks ;-)

I'm really new to X1...and this has happened to me as well..... I don't recall exactly what I did to undo it, but it IS a setting.... something you accidentally clicked on or brushed the keys.... 

to test that it's not the DAW or the sound card or the drivers, simply close the project and open something else.  If it plays fine, the settings are off in the other project that suddenly won't play. 

I do this by accident every once in a while, and next time it happens I'll try to remember it.... but in the mean time.... play around and try a few things..... 

What's the worst thing that could happen? 

Ok, here goes. And for those of you thinking that I'm a dork, let me save you the trouble and confirm... that I em. (I don't dare use "I am" anymore while "bapu" is around... lurking).

Once I managed to calm down, I asked myself the following questions:

1] Is it happening only in this project?
2] Are they new projects created in X1 only?
3] Why those tracks and not others?

A) No, it's happening in other projects as well (again, some tracks were ok and others had the same symptoms).
B) No, some projects were created in Sonar 8.
C) All those problematic tracks had only 1 thing in common... the plugin.

I've been testing FabFilter products for a little while now, and the PRO-Q plugin which is fantastic btw had been used and abused in many different projects. The 15 day trial expired. And even though I was reminded on a regular basis on the # of days remaining to test it, it never dawned on me that this would happen at expiration time.

A bit pricey if purchased individually but I am saving up for the "Mixing Bundle".

I'm glad I did take a deep breath soon after panicking... and before I started to make an even bigger mess of things for something that was so simple to fix.

Hahahaha!! Good one!!!