Bizzarre audio loss

I was reviewing a half dozen timelines for sync issues.
I wanted to hear something better so I unplugged the cable to my sound and plugged in my headphones.
checked it with headphones, unplugged them put back cable to sound system (there was a pop)
now, no sound can be heard,
yes, meters show levels bouncing,
click to another timeline, sound can be heard.
back to the first timelin, no sound can be heard.
copy that timeline and paste on the one I can hear and I head it there.
back to original timeline, can't hear it.
crate a new timeline, and paste it. can't hear it.
play the source media outside of davinci in quicktime, I hear sound fine.
I open a third timeline and it's gone quite as well. this had sound 10 minutes ago.
I open a fourth, now it's gone quiet. it's spreading??
all along meters show sound.
now only the one timeline, the second one I opened, plays sound.
I feel like I have entered the twilight zone.
any ideas as to what i going on here?
Full disclosure, I have only been playing with Davinci for a few months and relatively clueless.
I wanted to hear something better so I unplugged the cable to my sound and plugged in my headphones.
checked it with headphones, unplugged them put back cable to sound system (there was a pop)
now, no sound can be heard,
yes, meters show levels bouncing,
click to another timeline, sound can be heard.
back to the first timelin, no sound can be heard.
copy that timeline and paste on the one I can hear and I head it there.
back to original timeline, can't hear it.
crate a new timeline, and paste it. can't hear it.
play the source media outside of davinci in quicktime, I hear sound fine.
I open a third timeline and it's gone quite as well. this had sound 10 minutes ago.
I open a fourth, now it's gone quiet. it's spreading??
all along meters show sound.
now only the one timeline, the second one I opened, plays sound.
I feel like I have entered the twilight zone.
any ideas as to what i going on here?
Full disclosure, I have only been playing with Davinci for a few months and relatively clueless.