newbie audio Q regarding playback/rendering

Hello all. My first question: I'm in the middle of cutting and suddenly audio playback on the cut/timeline side doesn't work. When I open the mixer I see the levels jump the way they should. I didn't do anything (consciously at least) to mute playback.

The red line going through the tracks indicates that I should render (which I've been doing with success all day -- same with video). No results from "render both" or "render all."

My inuition tells me that this has something to do with playback preferences or that some playback tab is not checked, but I can't find it.

At the timeline track indicators, all are clicked green. to the immediate right is see tabs a1 A1 / a2 A2 but no lower case a3 or a4.

I'm working with 4 tracks on 5.0, G4OSX 10.4.8 This is a 10 minute piece, I have plenty of memory. Thanks in advance for any help you can impart. Be kind -- it's my first time.