No Audio Even Tho Clips Are Visible

[Ellis Beckwith]

Sometimes after freezing and unfreezing a track (midi or audio), even tho the clip is visible and is highlighted in the clip list, no audio is heard. This only occurs after closing and reopening a session.

Things I've tried:

Shut down everything and reboot
drag and drop clip from clip list onto current track and onto new track
uninsert the instrument and reinsert
replace XPand2 with MiniGrand
copy a clip from another track onto the offending track
playback engine is set at 1024 / 1gb cache
bribery...

I've searched this forum as well as the rest of the web and only came up with one similar problem, but it involved losing the clip entirely (not visible in the edit window; not listed in the clip list.

I haven't tried importing the clip from a backup copy yet, but I don't think I should have to; the clip is there in the timeline. I can see it/ I can highlight it, I can move it...everything but hear it.

Has anyone else experienced this?
Thanks.

iMac (mid 2011)
Model Name: iMac
Model Identifier: iMac12,1
Processor Name: Intel Core i5
Processor Speed: 2.5 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 6 MB
Memory: 16 GB
Boot ROM Version: 83.0.0.0.0
SMC Version (system): 1.71f22
Hardware UUID: 08EC44C7-DE0D-5B08-847D-25FAFE8BC8AF

Presonus 192 interface usb 3.0
1tb ext HDD for songs and sample libraries.
macOS Sierra ver. 10.12.6