What did they check? 

• the cables are connected to the correct analog Out ports?
• the Port has been instructed to play (tested with a radio source)?
• the volume level is set correctly?

I’m not sure what you mean when you say the Bose sound system ‘plays no sound sound on the Port’? The connection is audio-out to the Bose. Might you have the line-in/line out cables setup incorrectly for what you are perhaps trying to achieve?

Do you mean the line-out from the Port does not play on your Bose system, but it works fine with your Sonos Five? Can you kindly clarify, as your post is a little confusing, to me at least?

If the ‘Five’ is working okay with the Port line-out, then perhaps check the Bose i/o selection and volume/mute settings.

The OP says his Line-In is fine because it plays elsewhere (on his Five).

He should be hooking the Port up in just the same way as the Connect was. Line-Out to the Bose.

FWIW I don’t see a Five in his profile. Only a Play:5/gen1 which suggests an S1 system. In that case hopefully the Port downgraded its firmware on installation...

Ah yes the mist clears (slightly), thanks @ratty, I thought the OP meant they had tried the Ports line-out to the Five’s line-in (just as a test) and it had worked, but there are things in the post like ‘Bose playing no sound on the Port’ that slightly confused me too, but nevermind, I’ll just step-out and observe.

Let me clarify:

Yesterday I upgraded my old Connect to a new Port and the old Play 5 to the new Five. I also upgraded from S1 to S2 app. (I also have two Ones that I have since some years.)

Line-In from the TV works fine; I get sound on all Ones and my Five. I unplugged the Audio Out (to the Bose) from the Connect and plugged it into the Port. So, the configuration is exactly the same, even the cables. But the Bose (connected to Port Audio Out) does not play any sound (absolute silent). 

Sonos customer support asked me to check that cables are properly connected and run some diagnostics confirming Line-In works correctly. They insisted I can’t use the Port, but have to get their Amp or replace my Bose with other Sonos components. But before upgrading, another representative told me that I need the Port, because my Bose already has it’s own amplifier and works with the old Connect.

Probably dueling volumes.  Set the Port to fixed output. 

Is your Bose amp switched to whatever input/port the Sonos Port is connected to? What is the audio source when your trying to send audio to the Bose?  TV, streaming service?

Distinctly unhelpful. 

A classic “loopback” test you can do is to wire the Port’s analog outs directly to its own analog inputs. Play something on the Port, an internet station for example. Then tell the Five to play the Port’s Line-In. If all is well you should hear the station. If you can’t, and after trying different cables, then the Port is probably faulty. 

This solved the problem! I did not know that the “Line out level” had different options, but after this comment I found them in the app. I would expect Sonos customer support to tell me this. My “Line out level” was set to automatic. After changing it to fixed I hear the sound on my Bose.

We can close this discussion. Thank you everybody! I greatly appreciate all your comments and help!