Are you sure the optical cable is fully seated at both ends? Make sure connector is oriented correctly and push firmly. It should click into place. 

yes the optical is properly seated and it plugs into the hdmi cable to the arc speaker.  

Why did you need to use optical?

i have an older tv and it does not have hdmi arc.   

Since you’re using optical, there’s a couple of fairly standard things that should be mentioned:

1. Make sure you’ve removed the ‘caps’ from the ends of the optical cable. I don’t think this is your particular issue, as I think you’d have no sound, not interrupted sound, but I’ve put this here for completeness.
2. Optical has no way of communicating back to the TV set. This means that you need to set not only the TV’s audio settings to be Dolby Digital only (no plus, no Atmos, no DTS) as well as all the audio settings for any device that you have hooked up to the TV set. It’s possible that there’s brief attempts to send something that doesn’t fit in the bandwidth available to optical, which would appear to you as a dropout. Setting those restrictions in the TV/connected devices keeps that potential from occurring. 

Hopefully, this helps. I’d be awfully tempted, the next time it occurs, to submit a system diagnostic within 10 minutes, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.