Hi Penny,
Fred's suggestion is a good one .
It will give you the functionality you require, the upgrade path/switch from JB7 to B2 is a well worn one and there is a lot of documentation and help available.
As Paul says Bluetooth may not work in your environment because of the distance and walls etc. There do exist wifi transmitter/receivers but they are few and not really designed for what you want to do. Any transmitter/receiver pair would require that your Sonos speaker to be have line-in connectivity and that is not true across the speaker range. I think currently only the Sonos 5 offers line-in.
I think there are some alternatives.
1. Sonos are launching the 'Sonos Port' which looks like it would be able to to make your JB7 visible to your network and Sonos as a source though playback control and choice of track would still be with the JB7.
2. You could add a NAS (Network Attached Storage) device to your network that your Sonos network could connect to. You would have to copy the music content from your JB7 to the NAS
and manually keep it in synch when you add any tracks to the JB7. This is really adding another separate music source and not particularly elegant.
It would require more set up and is a bit of a halfway house to a B2.
John