Try unplugging the Beam and Arc from power for a couple of minutes and reboot your router and phone.

Unfortunately not enough information to be confident, but it does sound like you have multiple issues going on. 

First, check the TV with the Beam connected to it for software updates, and apply them as necessary. Next, unplug both the TV and the Beam from power. Wait five minutes, then plug the TV back in, and wait two minutes before plugging back in the Beam. That should hopefully ‘fix’ the connection between the TV and the Beam, although it is still worth double checking the settings on the TV to be sure it’s locked down to Dolby Digital. You didn’t mention whether the connection was via ARC (HDMI) or optical. 

For the devices ‘disappearing’ issue, you could be experiencing wifi interference , or not, since we don’t know what device you are running the controller app on. At the very least, I’d delete the app from your device, and download a fresh copy, being sure, when prompted, to ‘connect to existing system’. 

Assigning reserved IP addresses to all network devices is a good way to avoid the potential of duplicate IP addresses being handed out by your router. A temporary fix is to unplug all Sonos devices from power, and reboot your router. Only after the router comes back up should you plug the Sonos devices back in, so they’ll get freshIP addresses from the refreshed router.

If none of these rather generic items resolve your issue, perhaps you should submit a system diagnostic within 10 minutes of experiencing this problem, 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. And there is no doubt more concrete data will help in identifying some, if not all, of the issues you’re experiencing.

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.