So…
All of a sudden, my recordings on channel 5-1 (CBS) no longer have any sound. I haven’t started troubleshooting it yet, but hoping to save myself some time and effort. Before I check a bunch of other channel recordings and live channels, and reset/restart everything, is anyone else experiencing this? 
4-tuner version, Android phone, Roku streaming devices, everything with the latest updates. Everything worked fine until 2 days ago. 
Recordings only, or live TV as well?
Is it this why on all your devices, or just the phone, or only certain Roku?
Lastly, do you have Audio Compatibility Mode turned on?
Thanks for taking the time to ask for more details.
I’ve had some time to troubleshoot a bit now and have some new info. But first, to your questions:
It appears to affect only recordings. Live TV has audio.
It only affects my Roku devices. The same recordings do have audio when played back on my ONN streamer’s Tablo app.
I did NOT have Audio Compatibility Mode turned on when the affected recordings were recorded. For the record, I NEVER had it turned on during the last year that I’ve owned the Tablo and used all of my multiple Roku devices without issue until a few days ago. I’m aware that some Rokus are not compatible with Tablo. I didn’t think any of mine fell into that category.
New info after some troubleshooting:
I turned on Audio Compatibility Mode and it seems to have fixed the issue. But nearly all of my previous recordings are now SILENT. They played just fine before. Really hoping there is a way to recover the audio tracks for these, but not optimistic. 
Weirdly, a few of the older recordings still have sound…no idea why.
Some of my Rokus no longer play recordings at all (no matter which channel or what settings are selected). They try to start playback, but then pop back to the Library main page. 
I have not yet tested all of my Rokus. I have six. Will get to it as soon as I can. 
Oh…I am not using an external hard drive, if that matters. 
What you have described is exactly what Audio Compatibility Mode (ACM) was created to fix. Some time ago, Roku pushed out an update that broke many, but not all Roku in regards to Tablo. They stopped supporting certain CODEX that Tablo needs. It didn’t just break sound, but would often crash the app or at least kick you out of what you were trying to watch. The problem did not effect ONN, Firestick, etc…
ACM changes how Tablo processes audio, and both displays live viewing using the compatible settings, and also records in these compatible settings.
So… recordings that predate turning this setting on will continue to be incompatible with impacted Roku, but new recordings should work on all your devices. They all should work on your ONN devices.
My only guess for the timing, is that Roku pushed out yet another update so that this problem suddenly hit your device.
Hope this is helpful.
This thread explains the problem and the solution Tablo came up with far better than I can.
Wow, great info. Thank you very much for your help! 
I checked and it appears that Auto Compatibility Mode is now ON for all of MY streamers.
I’ll read through the thread you provided and see if it means I will have to replace some of my older Rokus to fix the not playing recordings issue. At least I still have the option of watching the older recordings on my ONN streamer. It means I have to drag my wife to a different room than the one in which we’re used to watching TV, but that’s better than not being able to hear them at all.
Much appreciated! 
Happy to help. One last thing, while Auto Compatibility Mode is there really only to solve the problem with certain Roku devices, it is a global setting, not something that is ON for some devices, and OFF for others. Once it is on, it forces the Tablo (the puck) to switch to an audio format that all devices can handle, for both live viewing and recordings.
The only downside of ACM, as I understand it, is if you are a serious audiophile and want to use full surround sound. I am not, and do not. I have ACM turned on and find that it sounds just fine.
The Roku fix was for software version 14.X. More than likely your Roku is now running ver 15.X. I have two older Rokus that were running 14.X and I had Audio Compatibility Mode ON with Tablo and working OK. They were recently updated to ver 15.x and are working fine. I just left ACM on thing if it’s not broken don’t fix it. I also tried a new Roku Ultra and had problems with audio on all my streaming channels. Maybe it was my soundbar that was incompatible so I could not say Tablo was the issue. So I returned it and keeping my Roku 3810 until they die.
Yep, I’m on the latest OS for all of my Roku (I have six currently set up in my home). I’m pretty fanatical about keeping things updated. 
Good call with keeping your old Rokus. Every Roku I have ever purchased still works. I replaced a few of them with newer models because they are faster, but I have never had one die.
A couple of them are no longer compatible with the most recent OS updates, though. 
My understanding is that the problem introduced with Roku OS 14, no longer supporting Surround Sound audio (AC3), was continued in all later updates.
Last I heard, Roku has no plans to reintroduce support for AC3. While AC3 is the standard format for OTA television, Roku does not see themselves as an OTA device. The good news is that Audio Compatibility Mode is a good work around.