@Cor,
There’s nothing obvious that comes to mind, but the first thing I’d do is to simplify the setup since you have a number of moving parts.
The Combo modem/wifi/switch from your ISP is the first component I’d eliminate from the mix, followed shortly by the removal of the ZB. Since the ISP setup/provided the modem device, we likely don’t have enough information on how they’ve configured it internally.
The changes listed below are for testing/isolation. Once it’s been isolated to the specific component causing issues, then it would be built up again to get around those issues.
So my first step would be to cable both Vera3 and the Sonos ZB to different ports on the Cisco SE 2800. This way they’ll talk to each other without going through layers of switch gear (esp inside the modem).
Vera3 should be connected, via it’s WAN port, directly to the Cisco unit. None of the Vera3 switch ports (“1-4”) should be connected to anything. The Sonos ZB should similarly be connected to the Cisco SE 2800.
Devices should be power-cycled after you change the wiring “just in case” someone’s doing something odd.
If that doesn’t work, then we need to eliminate the Sonos ZB from the equation. In this setup, you’d directly cable the Sonos Play 3 to the Cisco SE 2800 (leaving the Vera3 on one of the other SE 2800 ports)
If it still doesn’t work, then I don’t have any extra clues to try out… since we’d have narrowed down to the most basic [wired] connections. Hopefully it won’t come to that 
Background questions…
In a normal setup the “ZB” will bridge Network packets, “destined” for a Sonos Play unit, over a proprietary Sonos Wireless network. You can directly cable the Play3 with Ethernet to avoid this config, and things will work just the same (Sonos units have a Ethernet and the proprietary Wireless network built in)
The “Say.nn.mp3” file is produced by the Sonos Plugin. It makes a URL call, over your Internet connection, to a Google TTS Service that takes our text parameter, and converts it to an Audio file. The Plugin “saves” this file on Vera, in a location that’s accessible via a URL, and then we tell the Sonos unit to “play” that URL. This all [usually] happens very quickly.