My setup with the Nav V, Motorola Atrix phone, and Sena SMH10 is working as designed. Your issue might be improper pairing of the phone with the Sena headset. First be sure to delete all pairings in the Sena and GPS. Also delete any paring in the phone for either the GPS or Sena. You need to start with a clean slate.
In order for everything to work, you need to pair the phone as "selective pairing" with the GPS from the phone selection of the GPS Bluetooth menu.. The phone default might be A2DP which will interfere with the A2DP pairing between the GPS and Sena. Pairing specifically from the GPS should solve this. Pairing your phone to the GPS as a phone only headset will afford full phone functionality. You won't be using the stereo media player of the phone when configured this way.
Next you pair the Sena to the GPS using the Sena to make the pairing. This will be a full A2DP pairing.
Using this method you will get media player of the GPS , GPS directions, and the ability to use the phone with the GPS controlling those functions. Everything will be prioritized nicely by the GPS. Be sure to have your Sena updated with the latest firmware and follow the directions for pairing from Sena and Garmin.
2013 R1200GS W
White, Enduro Pkg, Comfort Pkg, CC, Spoke Wheels