It’s such a potentially non-Sonos issue, it’s hard to tell. It could be the line in settings on the Sonos, as a remote possibility.  Unfortunately, it could be the cable you have, or an issue with the B&O system. 

If you have a Sonos device newer than the connect, you could try just using AirPlay 2 to it, and group the connect with it, but I’m guessing you don’t have one, since that would be ultimately better than trying to take an output from a B&O device and feed it to a Sonos system. Not overly compatible in lots of ways. 

Double check you are using the OUT on the connect and are using the aux on the 9000. There are a lot of DIN sockets on B&O.

If you have another device with phono / RAC outs you could try that on the B&O,  something such as a CD or DVD player.

This link may be helpful :

https://forum.beoworld.org/forums/t/36645.aspx  

Perhaps the pins in the DIN to rca connector lead are configured as an input, not an output?

In the post someone suggests pins 1 and 4 are used for the output with 2 as the ground.

Can you dismantle the RCA plug and see how it is wired? On some of those RCA plugs, the plastic cover slides off backwards if you press in a little plastic tab and then the plug splits in two. 

@ARjeily, @lohr500’s is almost certainly the right answer (except I think s/he swapped the words “input” & “output” in the 2nd sentence).  What happens on older B&O gear is that the preamp picks up the *crosstalk* from the output pins on its input!  So if you turn up everything to 100%, you barely hear a signal, just as you have described.  

Swap your DIN Aux-Out cable for a DIN Aux-In cable -- and turn down the volume! -- and you will likely hear your Sonos Connect on the Beosound 9000 just fine.  (Actually, you could buy a DIN Aux In&Out cable (also called Tape In/Out) thus you have all 4 RCA plugs, so you can play with them yourself -- and not rely on some vendor who doesn’t know his in from his out!:-)

Edit: @Belly M’s advice is correct too, but I presumed you filled the other two DIN sockets with Powerlink cables to your speakers, and couldn’t make that mistake.  If you are using the Masterlink socket instead, then yes, triple-check that you plugged into the correct DIN socket.  Because the 9000 can be mounted sideways and upside-down, the picture in the manual requires re-orienting, and that black raised plastic print on the panel cover is almost unreadable.]