A method for automatically switching to a channel for transmission on a portable radio is provided. In operation, a first microphone is assigned to respond to communications received on a primary channel and a second microphone is assigned to respond to communications received on a non-primary channel. The portable radio receives independent audio communications simultaneously on the primary channel and the non-primary channel when operating in a multi-watch mode. The portable radio determines a signal gain corresponding to a voice command received at the first and second microphones. When the signal gain for the first microphone is larger than the signal gain for the second microphone, the portable radio switches to a first talk-back channel to respond to communications received on the primary channel. Otherwise, the portable radio switches to a second talk-back channel to respond to communications received on the at least one non-primary channel.