Easiest solution..... Have the relay mute just the speaker. You will find, if your a "long winded operator" on upper bands, cutting the B+ sometimes causes the receiver to drift. I have done this (speaker mute) mute to a gazillion receivers without a problem ever.
Are you shorting or opening the speaker leads with this method? If opening I would add a load resistor to prevent excessive voltage from being developed by the unloaded output transformer and I wouldn't use the shorting method with a SS output stage without a series resistor. If the rest of the receiver is still active, then even with the antenna disconnected you are going to have a lot of drive to the audio stage with most receivers; very few have sufficient shielding to not pick up quite a signal while transmitting.