All of the Oticon Streamers are line of site and have a range of about 30 feet, however they seem to work the best in my experience from about 20 feet or closer, after that you start getting a degraded signal which sounds like white noise. Try plugging into the bottom of the Streamer with the patch cord and also plug into whatever your trying to listen to and see if you still get the white noise, if you do, the problem might lie in the Streamer itself. If the signal comes in clear you consider getting the Oticon TV adapter, it might work better than pairing it to your Sennheiser transmitter.
The Streamer Pro can not be muted. The one second push of the volume buttons only mute the HA’s and it will only mute the latest versions of Oticon HA’s, so if you have an older pair that might be your problem.
Tell your audi you want to try earmolds with an adjustable vent, since your the one paying they shouldn’t care. You could also try the double domes as Ken suggested and see if that works better.